Sorry about your loss.
I had a similar experience. My flame angel thrived for 3 months in my 500 gal aquarium, until one day, it refused to eat and then the next day, I found it dead! A closer inspection of the body showed that it's belly was bloated (I assumed at the time that it was probably constipated). The article mentioned something about the damage the stress does on the fish's digestive tract --- so now I'm thinking maybe it is related to what the article was talking about.
I never have luck with Flame Angels, so I've given up on this beautiful species.