During the Diablo fight, he transitions into that shadow phase, and then back out of it. I got stunned right before he transitioned out of the shadow phase. During the cutscene, my character was in a stunned position while my party was perfectly fine. After the cutscene, I noticed the stun debuff wasn't there anymore.

While stunned, Diablo used a spell on me, then attacked me, then put his fire circle under me, but none of these actions cancelled my stun. About 40 seconds later, he used his stun on me and then grabbed me, which finally got me out of the stun glitch.