I returned to playing WoW two days ago. I'm specced deep affliction. I noticed that Immo is kicking UA off my target. I try to recast UA and it says a more powerful spell is already in place. Is this supposed to happen or is it a bug?
Supposed to happen. Immolate and UA are now mutually exclusive, and in all cases immolate counts as the stronger spell.
If you're deep aff: Use UA not Imo.
If your're demon or destruction: Use Imo ( cant use UA cus its a talent )