I assumed it happened when someone on your instance subscribed to a new community for that instance so it loads in all the posts since they are technically new. Definitely a bug
Interesting take on it. Another bug that I see often is a post with like 5 comments, but when I click on "comments" I don't see any. Even mine in some cases! I'm not mad, I guess all of it will require some fine tuning to work flawlessly. Edit : I've found the problem. It the account settings I had unselected english in the languages list!