Avatar slouch when sitting/And Avatar brings both legs together when he stands still

Hello, as I did not find a topic about this problem that I have, I create it:

The avatar brings both legs together when standing still,
And later when he sits down, at first he is well seated, but brings his face to the front and stays that way
Video showing it: https://imgur.com/1Emy7dM

Is there any way to fix it?
Is it because of the new update, or something specific to this avatar?

(Desktop, I don’t have VR)