That isn't a bug, that is correct behavior under their contract model (which I will admit isn't my favorite).
It is common for message based systems for the target system to own the contract, and they have both the / and /v3/ endpoints that you can grab the version information from.
While I personally prefer the URL method, when versioning through custom headers, if you bump the API without that custom header, you will break way more than returning correct behavior for the minimum supported version, enforcing backward compatibility for API's is generally considered a best practice.
Note:
> If the OpenStack-API-Version header is not provided, act as if the minimum supported version was specified.
It is common for message based systems for the target system to own the contract, and they have both the / and /v3/ endpoints that you can grab the version information from.
This is documented BTW:
https://docs.openstack.org/api-ref/block-storage/v3/index.ht...
While I personally prefer the URL method, when versioning through custom headers, if you bump the API without that custom header, you will break way more than returning correct behavior for the minimum supported version, enforcing backward compatibility for API's is generally considered a best practice.
Note:
> If the OpenStack-API-Version header is not provided, act as if the minimum supported version was specified.
https://specs.openstack.org/openstack/api-wg/guidelines/micr...
Once again, fully documented, expected behavior.