We've found some challenges with transitioning from 2017-2018.  Today it's skylights.   Create a roof, insert a standard Velux skylight symbol from the default libraries and then display in 3D. It looks good, normal, etc.   Then create a viewport to display the roof with said skylight.  Even with every class turned on, the skylight disappears. Return to the drawing layer and it's back.  Enough back and forthing and the skylight will eventually disappear on the drawing layer as well.  It's still there ... mousing over it will reveal it's lines but for all practical purposes it's invisible.  It's not a class visibility issue so ....?   The problem persists in a new empty file.   Drawing   Viewport   Drawing (after a while)...   Using 2018 service pack #1 Mac Pro ....