Cannot Move Z Position on Precomp?

Hi gang!

Strange behavior. I have a precomp in a main comp where I would like to position it in Z space. If I change the Z value, only the anchor point moves (together with the XYZ - red-blue-green indicator) but not the actual precomp.
However, changing the X and Y, works just fine.

The precomp is marked as a 3D layer.

What am I doing wrong?

thanks!
gabi