Description of bug: I was running solo normal mode on my level 60 witch doctor, and I activated my spirit barrage - manitou skill during the diablo fight. After ending the realm of terror phase, I was ported out to the 3rd phase of diablo. However, at a point during the cutscene, my manitou spirit continued to shoot diablo (there were no damage numbers), and when cut scene ended diablo died immediately and loot dropped without his normal death animation. However, I did not get quest objective completion for killing diablo, and was unable to proceed with the quest.
I was able to replicate this with the following steps:
1) Cast Spirit-Barrage - Manitou during realm of terror fight.
2) Kill realm of terror diablo
3) Upon exiting realm of terror and entering phase 3 cutscene, press escape to skip the cutscene.
4) This results in diablo stomping the ground to mark the beginning of phase 3, but he dies instantly.
During Diablo's ground stomping animation at the end of the cutscene, my spirit barrage continued to shoot him.