When exporting a tube (i.e. closed extrusion) as STL with the following settings:
Maximum angle 0.0
Maximum aspect ratio 6.0
Minimum edge length 0.01
Maximum edge length 0.0
Maximum distance, edge to surface 0.0
Minimum initial grid quads 0
And check Refine mesh and Pack Textures.
I have a very simple .3dm file that can reproduce this problem, but can’t figure out how to upload it and attach it to this post.