What you see on the screen in Photos is a thumbnail or a preview version of a picture made from the original file. These smaller versions make it quicker to scan through lots of pictures. When you choose Edit mode, Photos uses the original file so you can see exactly what you're doing. It sounds like Photos has lost contact with the original files. When you copy the screen image, you're just getting the lower resolution version, and then you're importing that as an original. So what happened to your originals?
Photos normally stores the originals in its Library so that it can keep track of them--this is called a Managed Library, and it is the default setting. It's possible, perhaps for testing purposes, to tell Photos to not import the originals, but to reference their locations in Finder-- this is called a Referenced Library, and it's generally to be avoided, because if those files are moved Photos will lose track of them. Could this be what has happened with your older pictures? Perhaps an older version of Photos was set as Referenced. A new version always starts as Managed.
You can see the status now by looking in Photos' Settings>General:
(Photo's versions differ slightly.)
If Photos doesn't have the originals, then you may have to go hunting for them in old backups.
One thing that might help is to rebuild the Photos Library--close Photos and re-launch by option-command-clicking the app icon. If you use iCloud Photos, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.
Let us know what you find…
