You can work around this limitation by creating an iOS shortcut with the „Open in...“ function, setting the app to open the file in to Affinity Photo, and adding it to the share sheet.
Basically does what you are all asking for, don‘t ask me why Affinity didn’t add it to the Photos app. If you use the Files app, and select share on a photo, there is actually an „Open in Photo“ option which Affinity give us built in, which tells me they are still thinking of us using the program with a folder file management mindset. To be fair, for anyone working with large amounts of RAW files, using the Photos app would not be realistic, so working off/with external storage (and the then necessary Files app) makes the most sense anyway.
