This is a repost - I posted earlier in bugs, but this forum seems more appropriate...
How to make "Round-trip" edits from Apple Photos to Affinity Photo and back again:
In AP, open the desired Apple Photos image via the media browser (scroll down left hand side bar in "open..." dialogue until you see the Photos "Camera" icon, then select that, then navigate to your image for editing)
Note 1: if you double click here to select a photo for editing, it will not open in AP! - this may be a bug. But if you single-click (select) the desired photo then click the "open..." button, it will open in AP, ready for editing.
Edit away in AP.
When done editing, to save back into Apple Photos, just hit "Save...". The photo will be saved in its original file type on top of (overwriting) the original in the Apple Photos Library. You may also want to save a copy in Affinity Photo format to your desktop, just for safety.
BUT! Apple Photos will not yet know the photo has been edited, and will display a proxy image of the unedited original. Don't despair.
To quickly rectify this, find and select the image within Apple Photos that you just edited in Affinity, now click the "Edit..." button.
Suddenly your Affinity edits will appear, and you can now cancel out of the edit mode in Apple Photos.
Ta Da. There is the edited image in Apple Photos.
Note 2: If you try to "Save As..." from Affinity then you cannot navigate to the media browser. So you must "Save...".
Note 3: You may want to duplicate your photo first in Apple Photos before editing in Affinity, in case you want to save the original. Not sure if this will always work as sometime duplicates are only proxies of the original in Apple Photos.
Note 4: Affinity lets you "Save Editing History" but haven't tested if this works via the round trip method above.