I happen to notice that someone seemed to be able to downvote a deleted post.
How could this be possible? Or is it a system bug?
But you need to differentiate between downvoting a post, and an upvote being invalidated. Accounts get deleted all the time for various reasons (e.g. self-deletions), and when those accounts have not made "a lot of impact", their votes are invalidated.
I don't know if this is the case here, but that seems as the most likely scenario.