Colossus Fractals of the Mists
This is a bug. Happened to my team too. I think it has to do with killing Diviner before you kill the chains (Did this 3-4 times and the only time it bugged we got him down to 0 hp but chain was at like 20%).
Well chain needed 1 more hit, and he wasn’t dead yet. He just had a sliver of health left, still enough for prolly another minute of hard hits, but he was still alive.