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.