On several characters trying to farm the Heroic Deathwing mount, everything is fine until getting to Ultraxion.
The conditions needed for him to wipe out your action bar (primary, not secondary ones) are that he needs to drag you into the Twilight Realm.
I’m still able to auto attack and kill him, as well as use abilities not on my primary action bar and kill him that way. Killing him restores your primary action bar.
If it helps, I mostly use the default UI for combat, but it’s safe to also say I use addons.