Map boss disappeared mid-fight, unable to complete map

While fighting a map boss, the boss suddenly disappeared mid-fight.

The boss did not die, no completion was triggered, and the boss could not be found anywhere in the arena.
I left the map and re-entered the instance, but the boss did not reappear.

This prevented me from completing the map.

Screenshot/video attached.
https://drive.google.com/file/d/1rjK7dNlx6D0TilsBAgvlXCEFgIRxRwIc/view?usp=drive_link