Your file opens as a solid over here.....

That is even more puzzling, when I exported the part it was a joined srf with no obvious issues, but now when I open the exported file it too is a solid. I wonder why?
Thanks for taking a look.

Although I brought your file into another app and created an issue with this surface here:




Try replacing that surface and see if it helps you out...

I have, very rarely, managed to create objects which have some strange "leftovers,"
which when saved to 3dm and reloaded, are OK. (I know, it is a vague statement:-)
- Brian

Sometimes exporting as STEP and then reimporting fixes issues. Make sure you have some sort of scale set in your preferences before doing it.