The LISA documentation still references an older Azure schema where the hyperv_generation field was valid:
https://mslisa.readthedocs.io/en/main/run_test/platform.html#use-vhd

Using that field today results in a deployment failure:

Provisioning.smoke_test: FAILED   deployment failed. ValidationError: {'hyperv_generation': ['Unknown field.']}

which I think is related to this PR: #3990

To make the runbook work with the current schema, I had to update it from:

azure:
  vhd: $(vhd_image)
  hyperv_generation: $(hyperv_gen)

to:

azure:
  vhd:
    vhd_path: $(vhd_image)
    hyperv_generation: $(hyperv_gen)