Uber Sirius disappeared in the middle of the fight

Similar issue was already reported multiple times:
- https://www.pathofexile.com/forum/view-thread/2734984
- https://www.pathofexile.com/forum/view-thread/2718179
- https://www.pathofexile.com/forum/view-thread/3069235

Today same happened in my Uber Sirius fight (in the final phase). I've run away for a short while and when I came back to the fight area he was gone :( Just the flames and storms remained. I've recorded the video, as you can see he's nowhere to be found but the fight continues.

Video: https://www.youtube.com/watch?v=WExLteZU6yc