Adding a hinge joint had the unintended effect of picking up both the post and the spinning sign. The simple solution was to make the post static. I do wonder how I could make it possible to pick up the post and move it and then fiddle with something attached to the post without moving it…?

Another thing I did to make it work with the XR Grabbable system is that I switched the movement type from Kinematic to Velocity Tracking for the 2nd sign which seemed to maintain the hinge constraint better.