Very often (every time I encounter them?), Skullen mobs create a ‘blank triangle’ during a fight which causes my Shatter Strike visual effect to show the floor underneath.
Hard to capture an image mid-fight Skullen Blank Triangle.
It gets a lot worse than this with bigger triangles and when there are more than one there.
I’ve seen it from different variations of skullen, at all stages of the fights.
Noticed it a lot on the Shaman just before their lightning attacks, generally on the 1st attack.