Well, your "legacy non production test tenant" can be opened by just guessing passwords, and it allows access to "very much in use production non-test" tenants, then you could say MS has a vulnerability. It may not be a buffer overflow, but it is a vulnerability nonetheless.
Yes, and I think most people would consider it a vulnerability if an authentication system doesn't rate-limit or otherwise slow/stop "password spray" attacks.
You can rate limit individual users but password spray attacks use a large number of accounts to remain undetected in a authentication system used by an even more users.
