I was clearing Fields of Ruin with a guildie this evening when I decided to try using the Sentinel Rifle consumable on the Champion Branded Devourer event. The “One Shot” skill immediately took it down. I received no event credit for the kill, though I was able to loot its corpse and gained XP for all the trouble. I have not tried it on other Branded Champions, though I did manage to replicate the issue on another attempt on the Devourer.
While this issue is incredibly hilarious and has some interesting roleplay implications, I doubt it’s working as intended and should be addressed.