I discovered that whichever was the last character you played when you log out of the game, that is the character you will be posting on when you log in to the forums. And yes if you want to switch posting toons then you’ll be logged out.
So my workaround is to log out of the game on whomever I want to be posting with on the forums.
Blizz is aware there’s a problem and are looking into fixing it.