A user was just removed who had obviously voted on some of my posts. I lost 20 reputation which may not seem like a lot to all y'all with 400k+ (ahem, Valorum, ahem), but I was pretty annoyed. So what, exactly, is considered a "significant" reputation drop? It seems that the system would flag it. But where's the threshold?
We don't know.
The exact thresholds for things like what counts as serial voting, or automatic question and answer bans, are kept secret. If they were public, people might be able to abuse them. I've even heard of people abusing the system using things to do with account deletion, so that might be a reason to keep the threshold for vote invalidation after deletion secret too.
Or, there might be no fixed threshold. Deletion of high-rep or high-voting users is reviewed manually by SE employees, so there's probably an element of human judgement in whether or not to preserve votes.
In any case, 20 rep from a deleted user is no big deal. That's a whole 2-4 votes worth, currently 0.5% of your total rep, regainable with a single good or even not-so-good question or answer. (Maybe this sounds silly from me, sitting here with a 6-digit rep score, but all of that previous sentence is objectively true.) Speaking from experience, I don't think I've ever seen an account deletion costing any one user over 100-200 rep unless something else was going on, like a serial vote abuser deleting their account.