Here is how I got around this problem huge issue for me as I use my Mac almost only for photo editing
click to edit photo once in editing I chose external editor in the three dotted icon save in external editor I will take you back to photos chose external editor again photo should go to affinity edit photo close to save it will ask if you want flatted pic choose that your photo will appear back in external editor as you edited it save changes your photo should go back into Mac photos as u edited it in affinity this process is a bit of a pain in a......s not good for work flow but at lease in I can edit my photos and save to my library hope this help somebody I think it has to do with the flattening of the photos but not sure