In #171391 we had a situation where a number of analysis failures only showed up in post-submit when updating the macOS min OS version used for our deprecation detection. On further investigation, it was because the master test (which runs in both presubmit and postsubmit) passed, but the stable version of the test (which only runs in postsubmit) failed.

Maybe some auto-migration that happens on master invalidated the test in some way? This needs more investigation.