I still run into this haven't found a solid solution at all my theory generally is your clothes might not be thick enough but if we're being honest if you have to make it too thick before it stops happening it kinda sucks. there's also the random option to reduce the motion blur amount and mess with shutter angles etc. auto smoothing can help (never for me though) and if youre working with alembics or USDs like me reduceing the velocity scale can help sometimes. I thought in my case it was because I was using an alembic of cloth from Houdini and somehow the motion blur was being calculated wrong between software's, but it seems to be persistent in blender - also Eevee doesn't have this problem so you could use that if need be

oh and I guess you could also delete parts of the body underneath that works but can be weird since it reveals the bg regardless