I searched the US forum a bit hoping an answer was given there, and I found this : https://us.forums.blizzard.com/en/wow/t/world-of-warcraft-8-1-5-graphics-api-details/121541
Explains why some people say it works with DirectX11 Legacy. They made a change here. If you were using DirectX11 then the change can be the issue and you’d need Legacy one. If you were using 12 already, since they made “some improvement changes” to it it might just be the issue too. Hope it helped people in the same situation… Haven’t been able to test myself yet, but finally found a post where a blue has marked it as official solution for now :
https://us.forums.blizzard.com/en/wow/t/fps-drops-and-crashes/126446/3