You have created a design on your base surface, and when you show it in Model Viewer the trimming isn't applied or is only partially applied.

There are a few possible causes for this:

• Surface discontinuity - the trimming algorithm is expecting a single 'outer' boundary for the surface.  If your surface is not contiguous (there are multiple areas of triangulation with gaps between) the software will try to form a single boundary around all the triangles
• The Base Surface has been simplified - this can cause large triangles next to very small/sliver triangles on the design surface, which the software algorithm may fail to detect as a border
• The Base Surface has been set to 'fast draw' or cached - in these cases the boundary is either not applied or not updated following edits in the design