Same filter as Check 1: the red only narrows it down, the failure line decides. If the test fails on an assertion about the bug's behavior, it counts. If it fails because the revert took away something it needs, like an import, a helper or a fixture (the test_counts_at_runtime case), it's noise, not evidence.

When a whole-file revert breaks things that aren't the bug, revert less: undo only the hunk that fixed it, which is really the hand-written mutant from Check 2. And if the test can't even run without the new code, that tells you something too. It's tied to the new API rather than to the behavior, so I'd rewrite it against behavior that exists on both sides before trusting it.