The geometry may well have tiny gaps along an edge where two adjacent curved surfaces intersect. It may not be obvious when looking at the geometry in the parent CAD system unless it provides tools for querying and healing geometry defects. We occasionally see this problem when CAD geometry is imported from one system into another and the STL is then generated from the latter. Different CAD systems use different tolerances in their surface intersection algorithms. When the geometry is exchanged faces may not be stitched together properly, giving rise to gaps. The PDE toolbox will error if these gaps are detected on import.
If you email the STL file to me together with and any info on translation history then we can investigate; though at this point we cannot heal geometry with defects like this. email: firstname dot lastname at mathworks dot com
