I was able to reproduce the issue. The good news is, there's a workaround - go into Mod Settings and enable the Construction Planner mod setting "Auto-approve new ghosts". After that, any newly placed entities should be blueprintable and copy/paste-able as usual.
What's happening, as best as I can tell is this: both my mod and Creative Mod are listening for a ghost to be built. My mod gets to act first, and changes the ghost force to the 'unapproved ghosts' force. Then Creative Mod acts and replaces the ghost with a real entity. But Creative Mod doesn't know to change the force back to the main player force. There's no way it could know, and I don't expect it to.
What I think I can do, is maybe my mod can listen for any real entity somehow getting built on the unapproved ghost force, and just move it back to the main player force automatically. I'll try it. In the meantime, give the workaround a try, and if it doesn't work for some reason, let me know...
