@wim @erik6 Erik’s problem appears to be caused by unexpected behavior when Cplanes are created from a clipping plane widget.
When a Cplane is created from a clipping plane using Cplane with the Object option the Cplane is oriented with the clipping plane widget the first time a Cplane is created from the clipping plane. If the Cplane is rotated then any subsequent use of Cplane with the Object option creates a new Cplane the origin moved to the new center of the clipping plane widget but the new Cplane does not orient with the clipping plane widget. Rather it uses the orientation from the first time Cplane with the Object option was used with that clipping plane widget. This includes instances when a clipping plane is copied from an existing clipping plane and rotated. This behavior appears to have started in Rhino 8.
I started a new, more succint thread specifically about this unexpected behavior.