I’ve had this problem before and I’ve fixed it myself, but I happen to get this problem again and I forgot how to resolve it. Basically, rotating the lower torso rotates the base (I think), rotating the upper torso rotates the lower torso, and the head rotates the upper torso. Here’s a video to better explain:
All the parts are unanchored, the RootPart is set to the HumanoidRootPart, and I’m sure the Motor6D’s are set up correctly.
Oh I had this problem before, in the model try setting the primary part cframe to the humanoid root part and setting root priority of the humanoid root part to 1 to give it well priority.