>>388151857
It's because the vertices are still weightpainted to your bones. If you want to hide parts of your outfit then look into udim discarding, or separate meshes to a reasonable degree. People here get erect for one mesh avatars but it's actually not the most performant in real world. Shape keys on large meshes also are a huge performance hit. If you must use shape keys then you will have to shrink it in such a way that it follows your limbs from the inside (don't shrink the Z axis)