I too am having an issue with importing NEF files from my new Nikon Z 50 into Affinity Photo on macOS 10.15. When I view the photo in FastRawViewer, it looks fine (albeit, out of camera the colour is a tiny bit flat but this is expected to be corrected in post), however when I load it into the Develop Persona in Affinity Photo 1.8.3, it looks very desaturated, the contrast looks horrible, and it is very hard to correct it back to how the JPG/RAW preview looks. Is there something I'm not configuring correctly in Affinity Photo, or within macOS? I just switched from a Nikon D5600 and I was not having these issues with the NEF files imported from that camera, and none of the presets I've made from editing photos in that camera even come close to translating across to NEF files imported from the Z 50. I've attached some screenshots and the original RAW photo.
NEF File https://ln2.sync.com/dl/b4ec17690/xscdz8je-ddc7gjbd-bs8iw2kw-yttxe5qi
Screenshot https://ln2.sync.com/dl/4ac001e60/extizjrz-fm3ef4a9-5dcbbvm2-3kn93jgb