> “why is this broken test here that appears to test a behavior we don’t support”

Because somebody complained when that behavior we don't support was broken, so the bug-that-wasn't-really-a-bug was fixed and a test was created to prevent regression.

Imho, the mistake was in documentation: the Test should have comments explaining why this test was created.

Just as true for tests as for the actual business logic code:

The code can only describe the what and the how. It's up to comments to describe the why.