Drat! Oh-- you don't have to respond to each of us-- we both see all the posts.
Safe mode clears out caches and stuff, and it prevents the running of certain automatic processes. It may fix things. Or it might work in Safe Mode, and then, when you go back, it doesn't. That might provide some indication.
By the way-- do you use any "Cleaner" type apps? They are known to really mess up Photos databases.
Here are some other things to try on the Mac:
- Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
- Make a new user and see if the same problem recurs with it. This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder.
- Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.