I am having the same exact issue, I kill the Devoted Nightmare and pick up the essence, then lose during combat or if downed and have to restart the entire story quest. I’m tired of having to restart quests multiple times, and spend hours of extra time to complete them because of random bugs and glitches. Seriously, it’s nearly two months after launch and so many major and longstanding bugs/glitches still have yet to be addressed. Real sad.