Operating system: Multiple networked computers running Windows 10
Graphics card: Various
Short description of error
If you have a scene with locally linked objects or groups, save it to a different folder, then press File > External Data > Pack All Into Blend, then press File > External Data > Unpack into file (this is a way of moving textures from one project to another) any objects or groups that are linked with local paths no longer show up : the links are broken.
Exact steps for others to reproduce the error
Start with a file in folder A (which I'll be calling Group.blend)
Create a new file and save it as Scene.blend in folder A
Edit Scene.blend and link an object/group in Group.blend
Press Save As and save it as SceneB.blend to another folder (folder B)
Open up SceneB and press File > External Data > Pack All Into Blend
press File > External Data > Unpack into file
Save and re-open SceneB.blend
The object/group from Group.blend is no longer visible in the scene, as the link is broken.