Posted by Muzzer on 11/03/2018 12:39:16:
Most odd that the preview seems to show it hollow. And if you'd added supports, they'd be on the outside of the cup, not the inside. Somehow the model has become solid, albeit with the default (20%?) fill.
It means that the top surface is zero thickness. Odds are it's a problem with the model not the slicer.
One way of doing this is to subtract an object from another when they share a plane face – always more reliable to have the object clearly proud of the face so it has to penetrate it.
Another route is moving nodes on a 3D object and 'bending' a plane surface.
Sometimes it just happens…
Neil