please try this

That won't work as Hostname is already a pointer. https://github.com/gophercloud/gophercloud/blob/main/openstack/compute/v2/servers/results.go#L260

the testAccCheckComputeV2InstanceExists doesn't use microversion fetching the server resource. you need to create a new testAccCheckComputeV2InstanceHostnameExists and set service client microversion to fetch the hostname.

I've tested this as well by adding the computeV2InstanceCreateServerWithHostnameMicroversion here with the same result (nil pointer dereference). I've also added it as a test to testAccCheckComputeV2InstanceDestroy without luck.
Do you know if there's something else in the test suite that calls GET requests to the instance?

@kayrus I've attached the Terraform debug and trace log from running the TestAccComputeV2Instance_hostnameFqdn acceptance test. I do see GET requests to the /compute/v2.1/servers/<UUID> with the header X-Openstack-Nova-Api-Version: 2.1 but I can't figure out who's sending these. I hope you have an idea or maybe a pointer on how to track down the caller of these requests. Also here's the terminal output of the test run.

$ make testacc TEST=./openstack TESTARGS="-run=TestAccComputeV2Instance_hostnameFqdn -count=1"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./openstack -v -run=TestAccComputeV2Instance_hostnameFqdn -count=1 -timeout 120m
=== RUN   TestAccComputeV2Instance_hostnameFqdn
--- FAIL: TestAccComputeV2Instance_hostnameFqdn (33.28s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x102e29f20]

goroutine 4 [running]:
testing.tRunner.func1.2({0x10344e520, 0x10419f4a0})
	/opt/homebrew/Cellar/go/1.24.5/libexec/src/testing/testing.go:1734 +0x1ac
testing.tRunner.func1()
	/opt/homebrew/Cellar/go/1.24.5/libexec/src/testing/testing.go:1737 +0x334
panic({0x10344e520?, 0x10419f4a0?})
	/opt/homebrew/Cellar/go/1.24.5/libexec/src/runtime/panic.go:792 +0x124
github.com/terraform-provider-openstack/terraform-provider-openstack/v3/openstack.TestAccComputeV2Instance_hostnameFqdn.TestCheckResourceAttrPtr.func4(0x140005db9d0)
	/Users/user/go/pkg/mod/github.com/hashicorp/terraform-plugin-testing@v1.13.2/helper/resource/testing.go:1746 +0x30
github.com/terraform-provider-openstack/terraform-provider-openstack/v3/openstack.TestAccComputeV2Instance_hostnameFqdn.ComposeTestCheckFunc.func5(0x140005db9d0)
	/Users/user/go/pkg/mod/github.com/hashicorp/terraform-plugin-testing@v1.13.2/helper/resource/testing.go:1020 +0x64
github.com/hashicorp/terraform-plugin-testing/helper/resource.testStepNewConfig({_, _}, {_, _}, {0x0, 0x1400043b8a0, {0x0, 0x0, 0x0}, 0x140004277a0, ...}, ...)
	/Users/user/go/pkg/mod/github.com/hashicorp/terraform-plugin-testing@v1.13.2/helper/resource/testing_new_config.go:219 +0xd80
github.com/hashicorp/terraform-plugin-testing/helper/resource.runNewTest({0x103649648, 0x140002a28d0}, {0x103652b20, 0x14000003a40}, {0x0, 0x1400043b8a0, {0x0, 0x0, 0x0}, 0x140004277a0, ...}, ...)
	/Users/user/go/pkg/mod/github.com/hashicorp/terraform-plugin-testing@v1.13.2/helper/resource/testing_new.go:362 +0x1dcc
github.com/hashicorp/terraform-plugin-testing/helper/resource.Test({0x103652b20, 0x14000003a40}, {0x0, 0x1400043b8a0, {0x0, 0x0, 0x0}, 0x140004277a0, 0x0, 0x0, ...})
	/Users/user/go/pkg/mod/github.com/hashicorp/terraform-plugin-testing@v1.13.2/helper/resource/testing.go:979 +0x510
github.com/terraform-provider-openstack/terraform-provider-openstack/v3/openstack.TestAccComputeV2Instance_hostnameFqdn(0x14000003a40)
	/Users/user/src/labservices/ls-terraform-openstack-provider/openstack/resource_openstack_compute_instance_v2_test.go:901 +0x3c8
testing.tRunner(0x14000003a40, 0x10362e148)
	/opt/homebrew/Cellar/go/1.24.5/libexec/src/testing/testing.go:1792 +0xe4
created by testing.(*T).Run in goroutine 1
	/opt/homebrew/Cellar/go/1.24.5/libexec/src/testing/testing.go:1851 +0x374
FAIL	github.com/terraform-provider-openstack/terraform-provider-openstack/v3/openstack	33.641s
FAIL
make: *** [testacc] Error 1

The microversion on the compute client in testAccCheckComputeV2InstanceExists and testAccCheckComputeV2InstanceDestroy are only temporary and I'll create proper functions once the culprit of the error has been identified.

in resource.TestCheckResourceAttrPtr("openstack_compute_instance_v2.instance_1", "hostname", instance.Hostname), you're passing a nil pointer.

in testAccCheckComputeV2InstanceExists(t.Context(), "openstack_compute_instance_v2.instance_1", &instance), you're passing a pointer to a struct and then populate this struct with new values. the nil get overwritten with a new pointer, but old nil pointer was already passed to a TestCheckResourceAttrPtr func.

try this approach:

diff --git a/openstack/resource_openstack_compute_instance_v2_test.go b/openstack/resource_openstack_compute_instance_v2_test.go
index 7cc7fc87..06b0bced 100644
--- a/openstack/resource_openstack_compute_instance_v2_test.go
+++ b/openstack/resource_openstack_compute_instance_v2_test.go
@@ -910,13 +910,25 @@ func TestAccComputeV2Instance_hostnameFqdn(t *testing.T) {
                                Config: testAccComputeInstanceV2HostnameFqdnConfig(),
                                Check: resource.ComposeTestCheckFunc(
                                        testAccCheckComputeV2InstanceExists(t.Context(), "openstack_compute_instance_v2.instance_1", &instance),
-                                       resource.TestCheckResourceAttrPtr("openstack_compute_instance_v2.instance_1", "hostname", instance.Hostname),
+                                       resource.TestCheckResourceAttrWith("openstack_compute_instance_v2.instance_1", "hostname", testAccCheckComputeV2InstanceHostname(&instance)),
                                ),
                        },
                },
        })
 }
 
+func testAccCheckComputeV2InstanceHostname(instance *servers.Server) resource.CheckResourceAttrWithFunc {
+       return func(v string) error {
+               if instance.Hostname == nil {
+                       return fmt.Errorf("empty hostname")
+               }
+               if *instance.Hostname != v {
+                       return fmt.Errorf("unexpected hostname: %s", *instance.Hostname)
+               }
+               return nil
+       }
+}
+
 func testAccCheckComputeV2InstanceDestroy(ctx context.Context) resource.TestCheckFunc {
        return func(s *terraform.State) error {
                config := testAccProvider.Meta().(*Config)

Well, that's embarrassing but totally makes sense. Thank you for your guidance!
I've fixed the acceptance tests with your suggestion and they passed on a DevStack 2025.1 environment. I also squashed my commits.

doesn't the if gophercloud.ResponseCodeIs(err, http.StatusNotFound) { work here?

and how about this? let's omit info about microversions here

        flavorName := server.Flavor["original_name"].(string)
        flavorID := server.Flavor["id"].(string)
        if flavorName != "" {
                // Starting with microversion 2.47 Nova returns the flavor name but not the flavor ID anymore
                d.Set("flavor_name", flavorName)
                v, err := flavorsutils.IDFromName(ctx, computeClient, flavorName)
                if err != nil {
                        if !gophercloud.ResponseCodeIs(err, http.StatusNotFound){
                                return diag.FromErr(err)
                        }
                        // Original flavor was deleted, but it is possible that instance started
                        // with this flavor is still running
                        log.Printf("[DEBUG] Original flavor of instance with id %s could not be found", d.Id())
                }
                if v != "" {
                        d.Set("flavor_id", v)
                }
        } else if flavorID != "" {
                // handle case, when microversion was not set
                d.Set("flavor_id", flavorID)
                v, err := flavors.Get(ctx, computeClient, flavorID).Extract()
                if err != nil {
                        if !gophercloud.ResponseCodeIs(err, http.StatusNotFound) {
                                return diag.FromErr(err)
                        }
                        // Original flavor was deleted, but it is possible that instance started
                        // with this flavor is still running
                        log.Printf("[DEBUG] Original flavor of instance with id %s could not be found", d.Id())
                }
                if v != nil && v.Name != "" {
                        d.Set("flavor_name", v.Name)
                }
        }
        if flavorName == "" || flavorID == "" {
                return diag.Errorf("Error setting OpenStack server's flavor: %v", server.Flavor)
        }

No if gophercloud.ResponseCodeIs(err, http.StatusNotFound) doesn't work because IDFromName() doesn't return a ErrUnexpectedResponsecode like flavors.Get() does.
IDFromName() from gophercloud utils returns a ErrResourceNotFound{Name: name, ResourceType: "flavor"} with the name being the flavor's name.

Generally I like your approach more but have a couple of remarks.

        flavorName := server.Flavor["original_name"].(string)
        flavorID := server.Flavor["id"].(string)

This will always cause a type assertion error because either we have the ID (microversion <= 2.46) or the name (microversion >= 2.47) but never both at the same time. I'd suggest to leave it as is (flavorID, ok := server.Flavor["id"].(string)) but I'm open to other suggestions.

        if flavorName == "" || flavorID == "" {
                return diag.Errorf("Error setting OpenStack server's flavor: %v", server.Flavor)
        }

Same issue as above. Did you mean to write if flavorName == "" && flavorID == ""?

Further I think we have to keep the reset of flavor_id and flavor_name in case the flavor got deleted but the instance is still running. Otherwise it wouldn't behave the same as before this change.

I'd suggest to leave it as is (flavorID, ok := server.Flavor["id"].(string))

agree

Did you mean to write if flavorName == "" && flavorID == ""?

yes

Further I think we have to keep the reset of flavor_id and flavor_name in case the flavor got deleted but the instance is still running. Otherwise it wouldn't behave the same as before this change.

ok, I'll check this again in the next review cycle.