In T260485, Urbanecm noticed CentralAuth uses wrong ipb_by_actor values. That broke the block in some cases, if no corresponding row exists in the actor table. See T261325: Fix rows in ipblocks that point to a non-existing user in ipb_by_actor field, due to T260485 for fixing that.
IMPACT: As of now (2020-08-24), the issue is fixed in Wikimedia production, but there is still a lot (thousands?) of misattributed blocks. This means that when looking at Special:Special:BlockList, the "blocking admin" column would indicate a user who did not perform the block. This may cause confusion, especially in controversial cases.
We need to find a reliable way to fix them all, possibly via a maintenance script ran in someone's home, to not clutter git history more than necessary?
In case an actor row exists, it is however pretty hard to find the cases where it is the incorrect actor, and who the true actor is. If we know a row is wrong, we can try to find an entry in ipblocks on meta with the same ipb_address and a timestamp close to the local one. But how can we know whether the rows is wrong? Perhaps we could check whether the indicated actor even has (or ever had) the rights to perform the block. If not, the attribution must be wrong.