I am having exactly the same problem when importing NEF raw photos from the Apple Photos app. The files are from a Nikon D7000 which I have imported directly onto my iPad using a sd card reader. Landscape files are fine and import with the .NEF extension but portrait import with a .TIFF extension and look to be the lo res preview image created by the camera.
I have found a workaround, if I export the portrait NEF files from the photos app into iCloud (keeping them as raw format), then importing those NEF files into Afinity and it’s fine, so it seems afinity can correctly process the RAWs but not when imported from apple photos.
