I, too, have periodic problems with NX. I don't seem to have problems with core IPTC meta data, but I have odd things happen with star ratings. For example, I will rate an image with two stars in PM. I will then view the large preview in PM, then hit the Edit button in PM to launch NX. I edit and save the photo in NX. I exit the photo in NX. When I return to PM, the star rating is gone. Happens a lot.
What settings are recommended, vis a vis saving metadata in XMP sidecar files, etc.?