Strange behavior during Sync Pose operation

[brothermechanic]

Hello
Help me please with this issue
I want to import character in blender pose with action and rest (default symmetrical) poses, need for posttweaking mesh with mirror modifier
But there is a problem
I did screeencasts
Video.zip

I'm not interested in face, I'm interested in body distortion

May be there is another smarter way to do that
Give me advice please
Thank you.

[Head-Hunter-is-OP]

bandicam 2020-12-07 19-35-24-440 (2).zip

I think it may be that pose which is awkward.
export the pose first