This is a very old issue, and to solve this shadow, try to solve it with materials or merge the polygon itself.
However, if you look at recent games, even characters with separated parts continue well to the areas where the shadows are cut off.
After that, as the character’s tightening parts, costume changes, and face animations increase, the part is purposely cut off due to the issue of capacity increase.
Should the meshes in this cut area be modified using the material of the normal?
Is there an unreal learning example that solves this issue?