This bug has been around in other places (and possibly here) for at least a year or more, but the graphics around Kel’thuzad when he goes into vengeful destruction mode (with downstairs phylactery element) in Sanctum of Domination display as a large black square. It was not like this when the raid was live and I recall some island expeditions having this issue in the past year or so where there are necromantic ritualists.
https://i.imgur.com/wiwM5GA.jpeg