I can try and upload a RAW file and it's corresponding JPEG later but as I said above, I don't think it is the file that is the issue, it is the route that it is taking - i.e via the import process into PHOTOS app on iPad. AP obviously has no issue with the RAW files themselves if they are loading directly from iCloud and opening fine.
The location (https://www.rawsamples.ch/index.php/en/nikon ) mentioned above would probably give you a file to try but I suspect it might only be an issue if PHOTOS imports the NEF and JPEG file at the same time which appears to be the default behavior when the camera is outputting RAW+JPEG.
Certainly, it would be interesting to know what the result is if the camera is set to RAW only, If there is no corresponding (they're named the same apart from the suffix) JPEG file, will it load from PHOTOS? If so then I suspect it is trying to load picturename.jpg when it should be picking picturename.nef - that would explain it thinking it is the wrong format?