I think he means it instantly going back to full. It's happened to my group too. While you are shooting him it instantly goes back to full and stops taking damage. Seems to happen mostly around the 25% mark.
The most annoying thing is crota standing back up after not even being down one second and killing our swordsman.
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Hmm, I've seen it, but I still think it only happens when you leave him undamaged a bit too long. Even if it's only a second or two. It's a God after all...