The problem is that we can't reproduce it. Otherwise we would have
already found the problematic commit (yes, git bisect would be able to).
It probably only happens with lagged slaves and many people editing at
the same time.
I have already studied the commits in that range, but none of them seems
likely to have produced the bug. :(
As we can detect it easily on big wikis (such as enwiki) what we could
do is to (a) rollback enwiki to such version, (b) verify it got fixed
(ie. it's a mediawiki core/extension bug), (c) advance bisecting to the
next wmf release.
Yep, bisecting in the live cluster. That's far from ideal, but not being
able to find it otherwise, that _should_ reveal it.