Still not fixed, just incase anyone was unsure.

No idea what's causing this but i have now run 2 separate t15maps with my frost deadeye and both times The black crow has skipped his transition, become immune to damage and i have to just go to a different map, i saw a few others mention something similar a few weeks ago so wanted to make sure its still up here as something that needs to be looked into cause its getting a little frustrating at this point, im unsure if its even worth running blooming fields.

It might be that your dmg is high and you overdps his phase change to much. You could try to dps him slower in first phase. Its a bug tho but might be avoidable