I get odd issues like this as well. Directly setting a Player’s Character property to an object sometimes works and others doesn’t. In other instances, I have other weird issues.
It’s Roblox messing up character physics for the umpteenth time. Try setting rootpart density really high and everything else really low. Make sure the rootpart is fairly large as well.
Really sick of fighting with humanoids doing bizarre random nonsense. I have a large character who routinely suddenly starts spinning uncontrollably.