It appears to be a block, I'm afraid. When you block someone (or you are blocked by someone) whom is on your friends' list, you do see their dot on the heightmap, but no characters or posts from them.
It doesn't look to be a glitch, since we haven't heard of any accidental blockings or unintentional kicks from group chats as of late.
However, if those users are blocking you for no reason, your time is probably better spent around people who appreciate you more. o: I wouldn't sweat over those guys.