About model and data exchange with 3rd party solutions: Revit, Solibri, dRofus, Bluebeam, structural analysis solutions, and IFC, BCF and DXF/DWG-based exchange, etc.
No idea why this would happen. Perhaps try deleting and placing again?
BTW: You should put your hardware & software info into your signature (click on "Profile" above). Problems are often specific to particular configurations or AC versions.
Point taken Matthew. I've updated my profile
anyhow
the problem is one in the office (not a personal one! - 😉)
in AC12
and I'm beginning to suspect a display problem as it may be affecting PDFs too - I have just learned...