When a spam or rude/abusive flag is raised on a post, a downvote is automatically applied from the Community user. This downvote immediately affects the post's score upon the flag being raised.
When a moderator clears (disputes) the spam and rude/abusive flags on a post, the downvotes from the flags are immediately removed, a score recalculation is triggered, and the new score is sent out to the SE WebSocket for all users who have the question page open to get an updated score.
Bug: Declining a spam or rude/abusive flag doesn't trigger post score recalculation
Unfortunately, when a moderator declines a spam or rude/abusive flag, while the downvote is immediately removed in the system, a score recalculation is not triggered for the post. The post's denormalized score is not changed. The new score is not sent out via the WebSocket (there is no new score to send, because it hasn't been updated). If you reload the page, you still see the score with the downvote from the declined spam and/or rude/abusive flag(s) applied. If nothing is done, then it can take many hours prior to the score being updated. I've personally seen it take 12+ hours for the score to be updated by the system. This is inconsistent with the behavior for clearing (disputing) red flags, which immediately updates the denormalized score.
A score recalculation for the post should be triggered immediately when a spam or rude/abusive flag is declined.
Not immediately updating the post's denormalized score allows some of the harm from the declined spam or rude/abusive flag(s) to continue. This seems to be a bug, as there doesn't appear to be a reason why the score wouldn't be immediately recalculated when a spam or rude/abusive flag is declined.
Workarounds: load the score breakdown or quickly vote then unvote on the post
If someone loads the score breakdown, or votes-up/down the post, then the post's score is immediately recalculated, no longer including the downvote(s) from the declined spam and rude/abusive flag(s). So, in order to immediately remove the lingering downvote from a declined spam or rude/abusive flag, then you can click on the post's score to load the score breakdown or vote. If loading the score breakdown doesn't cause a recalculation and you don't want to leave an upvote or downvote, then you can click on either the upvote or downvote button and quickly click on the same button again to return your vote to what it was. Having your vote briefly applied/removed will still result in the post's score being recalculated without the declined spam and/or rude/abusive flag(s). Voting, of course, requires that your up/downvote for the post is not locked in.