Posts can be re-flagged and re-hidden after a moderator has already reviewed the post and restored it.
If a mod has already let the post stand once, there’s no reason to do it all again. It’s a waste of time and serves no good purpose. It was reviewed and found not to be a violation already.
This is the part of the message you receive after a post is re-flagged:
Please fix this bug, which will also help discourage the use of the flagging system as a means of downvoting.