Thanks for your thoughts. It was something I have been trying to figure out for some time. It's not a major problem as I can edit on my Moto G5 and Win 10 laptop. At some point I will have to update my tablet.
Cheers, Kodak2.
A little followup -- I just experienced what you were describing, and for me, it was because the photo I was trying to edit was one I initially downloaded. Here's the sequence of what happened to me:
1. Downloaded a photo that was attached to an email from a friend.
2. Used a file manager app to copy that photo to the /DCIM/Camera directory so that it would get automatically uploaded to my Google Photos in the cloud. (I don't have Google Photos set up to automatically back up the Download directory.)
3. Opened Google Photos app and tried to crop that photo, but noticed that after I saved it, it didn't show up in Google Photos with the crop edit.
4. Opened the file manager again, and noticed that the edited photo was actually in the /Download directory (where I downloaded that attached photo in the first place). So although I thought I was editing the photo in the cloud, the Google Photos editor was actually editing the copy in the /Download directory, and then saving that edit as a separate file.
5. Once I deleted the original downloaded pic from the /Download directory and then went back to Google Photos to edit the photo, then the edit saved properly.
See if that corresponds to what your situation is.