Doing Fractal 12, we wiped on the Legendary Archdiviner in the Colossus fractal. When we came back and started the fight again, he was stuck in Phase 4 for the entire fight. Though he didn’t start doing his agony attack until actual Phase 3, he spent the entire fight summoning his scorpion adds and putting people in cages (well, largely putting the same poor guy into a cage over and over and over again).
This happened for two straight wipes and restarts until our group leader had to leave, so it appears that once he’s stuck like that, he never resets back to normal.