From What does “user was removed” mean and why did my reputation change because of it?
The reputation adjustment always occurs whenever a user is deleted, unless that user has cast a very large number of votes. In which case, deletion will be delayed so staff may consider preserving the votes prior to the deletion. The decision is at the staff's discretion and cannot be reversed after the deletion has taken place. If you are seeing a "User was removed" event in your reputation history, it implies that the user either hadn't cast enough votes to be reviewed, or staff made the explicit decision not to preserve the votes.
Given the large rep impact on many users across the network, one can only conclude that this part applies:
staff made the explicit decision not to preserve the votes
Also, from Please update the text in help for “User was Removed” reputation changes:
Hitting either of those thresholds only guarantees someone from staff will review the deletion. It is not a guarantee that the votes will be preserved, and we may refuse to preserve the votes for any number of odd reasons relating to the account.
Voting fraud being involved is only one of the most common reasons that we would refuse to preserve votes. It is not the only reason, and voting fraud existing doesn't even always mean we would refuse - it's just incredibly likely that we would refuse. So to everyone: Please do not assume that votes not being preserved was due to voting fraud. It is not always true.
It seems they will not (likely cannot, for privacy reasons) be providing more detail than that.
Alternative database-specific interpretation:
It turns out that user was voting inside a long-running transaction, and we were all looking at our reputation score as calculated under
READ UNCOMMITTED isolation. The
ROLLBACK TRANSACTION is now complete, and any memory you might have of a higher reputation score reflects a transient inconsistent state of the database 😃