Mesh warping in pose mode

[oo_1942 (Kim)]

I don’t know if this will help solve the problem because I don’t know the current status.

Check the axis. You can check the axial direction by enabling the settings as in the image.

If the mirrored axis orientation is different, select Ctrl+a→Scale.

Mirrored method without affecting axial direction

Mirror Pose Problem of Wrong Axis Rotation, Is it possible to change rotation axis?

Mirroring depends on the local axes of the bones. The easiest way to get proper mirroring is enter edit mode, select the bones that you want to copy from, and use the symmetrize operation. This will ruin any existing animation on those bones’ mirrors. There is also a risk of it affecting constraints.