Mask Control of Contour Labels
Autodesk's Jeff Bartels shows how to to control Civil 3D contour label masking by object and by Feature setting.
From the Jeff Bartels YouTube Channel.
and the Civil Immersion blog.
It is easy to miss the reality that every Contour Label has a Contour Label Line component. The fix works whether you employ the Multiple Contour label tool or not.
From a publication and performance viewpoint you might want to Label the surfaces in separate drawings and publish them as stacked referenced DWFs instead of XREFs.