The problem could be due to directx 12 and some drivers. Maybe a bug.
I saw that VLC also has a similar problem, but it is solved by changing the hardware encoding from automatic to directx 11.
It is likely and explains why 5, the integrated player of ffmpeg and other players work perfectly, with excellent performance with any video.
Is there a way to force the use of directx 11 in mediaplayer via code or something else?
How can you use the vlc library in a Qt project?

@jsulm Ok I'll do. Thx for the advice.