Just adding that I am also experiencing this very frustrating bug. Happy to share system/software/photo library information with Affinity if they need more data to work on a fix.
I guess it could be relevant that Affinity Photo is not shown in System Preferences | Security & Privacy | Privacy | Photos and there is no obvious way to add it.
The workaround of committing an edit to a photo in Photos first before editing that photo in Affinity Photo works for me (2/2 so far). In both cases I chose Auto Red-Eye as an action that would have minimal or no material effect on the photos.
I assume this has always been the case, but with a photo edited in this way I can successfully do Revert to Original or Undo Edit in Apple Photos to undo the AP edit, but it's a one-way process as undoing either of these actions has no effect on the photo.
I have been editing jpegs only, so far.
macOS 10.15.7
Affinity Photo 1.8.4 (App Store version)
