When they come back, do you actually see the full photo file reappear in the /DCIM/Camera directory? Or do you only see it in the Gallery app? If the photo is backed up to Google Photos, then it will still show up in the Google Photos app (on its main screen), and if you're using a stock Samsung Gallery app, it may still show up if the app is synced with your Google Photos in the cloud. So basically, the photo is in fact deleted from your phone, but still shows up because you're seeing the cloud copy.