SlicerRT importer imports the original contours without any changes and stores it in planar contours representation. If you create closed surface representation then the original contours are still exactly preserved at slice positions but a surface is created between them by triangulation. Conversion from planar contours to ribbon representation was broken in Slicer-4.11-20200930 version but it is fixed in recent Slicer Preview Releases.
Andras, I did not do anything myself to create a closed surface representation - it is created automatically when I load RTSTRUCT (this behavior is identical both in stable and latest releases). Because the surface representation is created and shown by default, I was not sure what is being shown in the 2d viewers, and the discretization of the 3d surface was very coarse, which made me suspicious. Thanks for the clarification.
This is expected - you only see the contour in-plane if it is exactly in the slice view’s plane. This is exact match may be hard to achieve, that is why we added the ribbon representation, which is much better visible in slice views.
These features are somewhat hidden, but both planar contours and ribbon model are useful mainly for diagnostic purposes (when closed surface representation does not look good then you can check the original data).