Thanks for replying, Judy. I'm using Navisworks Simulate 2019 but I've noticed this problem for at least 4 years (from NW2015).
Ideally I would like to add the MicroStation files directly to Navisworks but I have had to go with the "File Exporters" option to overcome another problem. When I attach DGN files I will often find certain objects have moved or rescaled. These objects are cells (MicroStation's equivalent to AutoCAD blocks) and I believe is this may be caused by the working units in the cell being different to the working units of the DGN file they have been inserted into.
Attached are two example screen captures I've taken showing my 3D standard man. Number 1 was exported from MicroStation using the NWEXPORT16 mdl application. The skin colour (RGB 255,205,148) has not exported to Navisworks at all. Number 2 is the same DGN file attached to Navisworks. You can see that the skin colour is fine but his helmet has disappeared because it is a cell. Usually these cells have either moved or rescaled by 10, 100, etc.
The RGB is a problem I can overcome (although it is cumbersome) but I can't overcome the disappearing cells problem. So I don't attach MicroStation models directly to Navisworks.
I'm hoping you might be able to offer a solution to either of these problems.
Regards,
Grant Tilley