switching Automatic scale grid on & off, I discovered the Print-Preview screen is affected
to the point 80% of the drawing lines disappear !
coming back to the Application Preferences panel I witnessed that simply opening and closing it
triggers the bug which almost erases the preview screen...
note that the bug only performs if and only if Print-preview is active
when closing the panel. You must close and reactivate Print-preview to see all the lines :)