there’s a bug when you place unanchored parts as a child of a character (it seems to fling more with 2 or more parts), and when the player jumps, it flings them. Is there any fix to this?
dont put unanchored parts in the player.
that being said, it happens because it messes with the models bounding box, physics, collisions, etc.
basically: just dont do it 
if anything, use a rope or make the parts collision group differ from the characters other parts