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.
Planar contours:
Ribbon model:
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).