AK Hard - Anchag bugged?

While doing Anchag (first boss in Amdapor Keep Hard), we were progressing as usual, one statue down and the boss at about 20% or so when all of a sudden, it healed back to 100% HP. Never seen this and figure it must be a bug so reported it in-game.

Has anyone else seen this? Is it a real mechanic?

I've fought him twice and didn't see his HP restore either time. Probably a bug.

I haven't experienced this either, but early days yet. I'm pretty confident it's a bug. First time in there my group practically ignored all mechanics.