I checked it on phone 2, working smoothly. 2160p 60 fps, 1440p 60 fps, 2160p 120 fps. No stuttering at all. Just received an YouTube app update, so try it once and let us know if the issue is solved or not. Sometimes we need to clear app data in the YouTube to solve HDR and stuttering issue. I did it just after A14 update. From then not required.