Instance blocking only blocks communities on the instance, not the users. If a user from the blocked instance posts to another instance you don't have blocked, you still see the user.

That said, I have also noticed USERS I have blocked getting unblocked recently, and they are not simply on an alternate account.

Edit: It seems the users I am seeing become "unblocked" is actually an asshole going around impersonating other big accounts (mainly mods and admins). Has nothing to do with the app.