1. Create a post and tag, for example, @Kier
2. Now edit your post, you'll see something like this:
If you remove @Kier and instead enter @Mike the system will show you the users whose usernames starts with "Mike" and then you choose the new user you want to tag and it's inserted correctly on the editor however the userid of the first user (Kier) is still there. This sounds like as designed or a technical limitation however I'm still reporting maybe it's a bug.