Submodel does not show properly in LPub3D
It looks like I might not be doing anything wrong after all. If I switch the preferred renderer from native to LDView under Preferences everything looks fine.

Maybe I should mention that I am using LPub3D 2.3.6 under Linux. At least the Linux version seems to be rather buggy. It regularly crashes with a segmentation fault when you try to edit the configuration. So this behavior might be a bug, too.