If have a model in millimeters. If I import a cad background in meters, I have to set it x and y size to 1000 to convert the size. That works.
But now the background disappears whenever I pan and its origin gets out of view.
I recreated the problem in a blank model with the default ER_small CAD file and it does the same. See attached model.
EDIT:I think it is a feature and not a problem. Perhaps you get around if you deactivate perspective view option. 
It only does it when the model units are mm and you have to scale the layout. I am reusing an old model so I can't change the model units. I solved it by scaling the new CAD beforehand in AutoCAD. So problem solved. But it sure looks like a bug.
"The drawing's scale factors affect the view culling of the DWG renderer.
If the scaling is uniform across the X, Y and Z axes then the drawing should not clip inappropriately when the view is rotated or zoomed.
If you need non-uniform scaling, rescale the drawing in AutoCAD."
From: Importing CAD Drawings and Floor Plans (flexsim.com)
If you change the Z scaling in your sample model to match your X and Y scaling, then this view culling issue is resolved.
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