I have the same problem and can confirm that Affinity Photo does not show full resolution on very high resolution or simply large files.
We regularly process very large paper sizes. In our test today with a PNG file of 30669 x 16457 pixels (410 x 220 cm @ 190dpi), the full resolution is shown when the PNG file is first loaded.
If you then save the file as .afphoto or TIFF and open these files again, the full resolution is no longer displayed.
If you open the TIFF in Apple Preview or Adobe Photoshop, you will see the full resolution again.
So the screen display of Affinity Photo cannot be trusted. This makes the further use with us more than questionable. Can this be remedied?