So far it’s seen spell texture problems in two raids from Shadowlands.
1st - Sanctum of Domination ~ Kel’Thuzad [phylactery portal casting the whole floor is a black texture]
2nd - Sepulcher of the First Ones ~ Just after the first boss in the Immortal Hearth, the ball of power that keeps coming down the hallway has the same black square floor texture.
Done the usual reset all graphics and games settings to default as well as GPU driver reinstall but nothing has fixed it.
Shall keep an eye out for more as I progress as never happened before.