As part of T376822, and especially T376822#10542726, I added explicit overrides for $wgAddGroups and $wgRemoveGroups to the WMF config. At the same time, I found T386210, a core bug which prevented $wgAddGroups and $wgRemoveGroups entries from extension.json files from being properly merged in.
In T376822#10544346 I theorized that the production bug was due to the WMF config setting, instead of merging, the $wgAddGroups and $wgRemoveGroups arrays. However, on second thought, this shouldn't make any difference, because settings from extension.json files are applied after WMF config. So, my current thinking is that T386210 wasn't a related bug, but rather the primary cause of the issue observed in production. If this is indeed the case, and local testing seems to confirm it, we will be able to revert r1119136, removing the WMF config overrides, as soon as wmf.17 reaches all wikis. If not, this task can be closed as invalid, and its author be told that he has a poor understanding of how MW config works.
