Have you tried to round trip the file to DWG and than back to DGN. This is kind of the opposite of what I found when I needed to dumb down some Civil 3D objects for MicroStation. Until I Round Tripped them, the software would not allow me to drop the graphics.
I think I even tried to use Merge into master to get them from a reference file and it would not bring everything.
I think I even tried to use Merge into master to get them from a reference file and it would not bring everything.