Well that's bad design. Google Photos offers to back up your photos, and also offers a way to free up local storage by deleting the older backed up photos.
Except by default it doesn't back them up at full quality and it doesn't warn you of that on cleanup as far as I can tell. So oops the originals are gone now.
@JulieSqveakaroo Fortunately I noticed the bad design when poking around the app, not after a loss!