Hello
Maybe some vertices weighted to zero at a point they need at least a minimal weight to follow the rig. A weight to 0 is already shown as assigned to a vertex group. Maybe you have some vertices weighted to a bone thats out of your focus… maybe a bit of a leg to a finger…
You possibly have weighted and exported your IK bones?
Test out the different export, import options.
Take a look to your bone rotations and maybe do some changes in the bone Translation.
Unfortunately theres a huge amount of error sources from scratch.
Head up 