glance md-tag-create-multiple overwrites existing tags

Our md-tag-create-multiple (/v2/metadefs/namespaces/{namespace_name}/tags) [1] API overwrites existing tags for specified namespace rather than creating new one in addition to the existing tags.
Where as if you try to create different tags using md-tag-create (/v2/metadefs/namespaces/{namespace_name}/tags/{tag_name}) it is working as expected, means adding new tag in addition to existing ones.

Steps to reproduce:
1. source using admin credentials
$ source devstack/openrc admin admin

2. Create new public namespace
$ glance md-namespace-create TagsBugNamespace --visibility public
+------------+----------------------------------+
| Property | Value |
+------------+----------------------------------+
| created_at | 2021-08-06T17:43:03Z |
| namespace | TagsBugNamespace |
| owner | a14a058e2d1540c3a0dc7c397c55174e |
| protected | False |
| schema | /v2/schemas/metadefs/namespace |
| updated_at | 2021-08-06T17:43:03Z |
| visibility | public |
+------------+----------------------------------+

3. Create single tag using md-tag-create command
$ glance md-tag-create TagsBugNamespace --name tag1
+------------+----------------------+
| Property | Value |
+------------+----------------------+
| created_at | 2021-08-06T17:57:37Z |
| name | tag1 |
| updated_at | 2021-08-06T17:57:37Z |
+------------+----------------------+

4. Create another tag
$ glance md-tag-create TagsBugNamespace --name tag2
+------------+----------------------+
| Property | Value |
+------------+----------------------+
| created_at | 2021-08-06T17:57:37Z |
| name | tag2 |
| updated_at | 2021-08-06T17:57:37Z |
+------------+----------------------+

5. Verify that we have two tags in the list
$ glance md-tag-list TagsBugNamespace
+------+
| name |
+------+
| tag2 |
| tag1 |
+------+

6. Add more tags using md-tag-crate-multiple command
$ glance md-tag-create-multiple TagsBugNamespace --names TestTag1141=TestTag2411 --delim =
+-------------+
| name |
+-------------+
| TestTag1141 |
| TestTag2411 |
+-------------+

7. Now run tags list command again
$ glance md-tag-list TagsBugNamespace
+-------------+
| name |
+-------------+
| TestTag2411 |
| TestTag1141 |
+-------------+

Expected result:
These new tags should have been added to existing tags.

Actual result:
Existing tags gets deleted and only newly added tags using md-tag-crate-multiple command remains.

* This is further to show that adding new tag using md-tag-create command now will add a new tag and does not overwrite existing ones.
$ glance md-tag-create TagsBugNamespace --name tag3
+------------+----------------------+
| Property | Value |
+------------+----------------------+
| created_at | 2021-08-06T18:12:14Z |
| name | tag3 |
| updated_at | 2021-08-06T18:12:14Z |
+------------+----------------------+

* Verify that we have not overwritten existing tags now;
$ glance md-tag-list TagsBugNamespace
+-------------+
| name |
+-------------+
| tag3 |
| TestTag2411 |
| TestTag1141 |
+-------------+

[1] https://docs.openstack.org/api-ref/image/v2/metadefs-index.html?expanded=create-tag-definition-detail,create-tags-detail,get-tag-definition-detail,delete-all-tag-definitions-detail#create-tags