Ah thank you so much. So it does open the dng if I go to Affinity, open, and select my file browser media menu and scroll through to the relevant photo. It just doesn't if I try to do this within Photos.
Does anyone know if this is going be fixed or if this is just how it's supposed to work? I don't think think I manage a workflow where I have to search through thousands of photos in a tiny window every time I want to edit one.
But if anyone is looking for the same answer Affinity will edit iphone 13 pro max proraws even though it's not on the list!
Edit: Though having said that this is so nice to use once you've got the photo open I might stick with it.