There was a thread on failing system tests with a few patches for fixing the issue sent, was it related to this?
It seems it didn't fail the upgrade job on the patch, so my suspicion is it might be related to the auto rebase done on merge.
We have an open ticket to enable Zuul for pre-merge tests (should actually run the check-merge jobs), that will help re-mediate such issues (by blocking the merge and giving -1),
We'll need to see if we can introduce it soon, after reviewing current infra tasks and priorities.