When Jaina was released and I finished it in 1 go, I didn’t bother to re-do a Jaina fight to get rid of that bugged notification on the Solo Adventure button until Rexxar was released.
Now it seems as a result of that decision I have that notification forever. The regular solution, re-fight Antonidas and then retire the run, doesn’t work anymore: it will temporarily remove the notification, but when I log back in it’s back. Also tried to re-do the Leoroxx battle just in case, which does the exact same thing.