This is not a support request to say but more of a bug report. During my expansion playthrough I faced a glitch with the bos of lvl 77 singleplayer instance. This didn't prevent me from clearing it, just broke my immersion a little.
Fortunately I have a twitch clip of what exactly happened: https://www.twitch.tv/videos/446377495 (sorry for commentary in Russian)
Please report it to the developers and have a nice day.