I had exactly the same issue, even after retreat. I took several tries, until I was able kill Scarif earlier than he got buff from Old Daka.
Simply, you must try harder ;-)
It’s Old Daka that is bugged, the issue has been pinned as a sticky (Bug with SRP in HAAT Raid)
I had the same in TW, but he had no buffs
Nihilus Killed him.
Also same problem if I have akbar in party.