Try and, reverse the parts to see if it could change because, most of the time when it does that its because the body part is being welded to the part instead of the part being welded to the body part.
Swap the part0 and part 1 of the motor6d, see if that works. If not, you could always just use a plugin to weld it. Personally, I really like the simplicity of https://create.roblox.com/store/asset/2908380847/Rig-Editor, so give it a shot if you need.
so asdthecoolest
Said
it was from the root priority
All u need to do if u faced this problem :
is to change root priority of the HumanoidRootPart To A Big Number or Just Scale it so big and make Can Collide off And transparency To 1