Hi rfields,
Below you can see that Affinity does return the edited photo back to Photos but in an obscure manner. This leaves the question:
How do you get to the edited photo once Affinity has returned it to Photos?
Here are the steps I took:
(1) Click on the thumbnail of the photo to be edited so it becomes large.
(2) With the cursor over the photo, right click to bring up the contextual menu. Move down to Edit with > then right and down and click on Affinity Photo from the popup submenu.
(I couldn't find any Edit in Affinity Photo as JFisher suggested, at least not under Photos Edit menu.)
(3) Affinity appears with the photo to be edited.
(4) Make edits with Affinity.
(5) Choose Save from the Affinity File menu. (A bit surprising since Save usually produces an .afphoto file on disk.)
At this point Affinity seems happy, but what has it done with the edited photo? It is nowhere among the master list obtained by clicking on Library > Photos in Photos.
By snooping around with FindAnyFile.app I located the edited photo in the directory path shown below:
So there it is safely tucked away somewhere within the .photoslibrary package.
How do we convince Photos to cough it up?