Sorry, I don't have time to make a file.
- Make an armature
- Link the cube to the bone
- Add them to a group.
- Save the file.
- Open a new file.
- Link that group.
- Make proxy of the armature
- Move the bone: the linked cube doesn't move.
- Save the file, reload it: the cube moves.
There's a similar bug, but in a different action. I'm reporting it, too.