I'd triple check on Mg, dKH, and s.g.
I've seen Meteor Shower do exactly that when those parameters were off.
Also, I've seen it where the tissue was gone, or barely attached, but the polyps were still inside the skeleton. And the coral came back after the water parameters were corrected.
Be careful though. If it does come back, the tissue is only very loosely attached for a couple months or so.