Pictures show up in Camera Roll but not in the Photos app

When you go to the Camera app, you can click on a thumbnail of the most recent photo and it takes you to a stream of all photos (I think this is referred to as Camera Roll). These have always been identical to what I see in the Photos app, but simply organized in chronological order.

I am helping out someone, so I don’t have the full background. They were saving photos from Google Photos. Those photos end up in the Camera Roll, but they don’t show up in the main Photos app. Does the Camera Roll have different permissions or something? How is it possible that a photo can be seen in Camera Roll, but not in Photos?