With drag&drop drawing import you'll only get model space content. Maybe you have paper space content only?
Or do you use the "Merge" dialog, which gives you further options?
EDIT:
I noticed that, as stated in the thread title, you can see the same merged drawing when imported into a worksheet? If so, then that's strange, as I never noticed such behavior.
As for 3D, as far as I can remember, merging a DWG doesn't import 3D data. If you'd like to import 3D content of a DWG file you'd have to import this file as a library object.
| Archicad 4.55 - 27
| HP Z840 | 2× E5-2643 v4 | 64 GB RAM | Quadro M5000 | Windows 10 Pro x64
| HP Z4 G4 | W-2245 | 64 GB RAM | RTX A4000 | Windows 11