After playing around with this for a bit, it seems like if I enable AvatarJointUpgrade (see here), this issue goes away for me. Something to note from the linked post:
AnimationConstraints do everything Motor6Ds can do and more. Motor6Ds will continue to work as they do today, but will not see improvements as they are legacy JointInstances. We will invest more in improving Constraint tooling and functionality. In the coming weeks, Motor6Ds will be marked deprecated in favor of AnimationConstraints.
Can you also give it a try with AJU enabled?