please take a look at the following face inside this 3dm file:
I convert the face into our NLIB-based format by getting the NURBS representation including the 2D U/V trim loops.
In this case, I get a single outer trim loop which consists of 5 edges. When converting this face to NLIB, we see an face that is invalid (smaller).
If I however export this face as a STEP file and re-import it into Rhino, the subsequent conversion to NLIB works correctly. Strangely, there are now 2 trim loops with each 4 edges.
I’m a little confused why the conversion fails with the original NURBS representation.