Poita: I agree, not having the saved edits be, well, saved (!) is a real kick in the teeth.
Not that you want to spend any more money to work around an app that should work, but, I have the app Photo-Sort. If I edit a photo in the Photo app, save it, open the Photo-Sort app and then download that photo to the Photo-Sort app, the edits "stick" [in Photo-Sort]. So, there's that.
I've just decided to continue to use Photo-Sort for my picture sorting, because I just can't get used to the new albums in the stock Photo app having a link to the original picture. Still too hard to look through the main album. And now, with this ... quirk ... that you found? Meh. No thanks. I like how Photo-Sort works to create new albums. Of course, you'd still have to delete the photos out of the stock Photo app, cause they'd be duplicate otherwise, but still.
Anyway, not an endorsement of Photo-Sort, per se. Just an observation of what I do as a workaround to your edit problem. Or, discovery, I should say. And - while I appreciate your annoyance at discovering this problem with the Photo app - I sure want to thank you for posting about it. Because, you saved me (and probably countless others!) a lot of time trying to figure out why edits aren't saved. So, THANKS!
Marilyn