The answer was likely removed automatically by the system after gathering a number of spam flags, as indicated by the Community user having the last activity on your question.
The denormalized answer count can get out of sync when this happens (typically when the user who posted the spam is also deleted), but it will eventually be corrected by an automated maintenance process that runs daily.
As for the inbox item, sometimes entries corresponding to deleted items aren't always removed. I've found this seems to often be the case if you've already read the entry, but that may have just been coincidence. There are a number of posts here on Meta about that topic, but I'm not sure if it's been determined conclusively whether or not that's actually the intended behaviour.