Some pictures in photos don't go full screen
I double click on a picture to have it fill the window and then arrow through more pictures, and each one fills the window, as well. But recently (months?) a picture will only half fill the window. This photo is not low resolution-- it's maybe 2000x 1500 pixels like the others, but it doesn't fully enlarge. If I hit the space bar it reduces to the thumbnail, and when I then click on it the picture will enlarge to full size. That picture may or may not do it again. I don't see any pattern in which photos enlarge and which don't, and it's rather irritating.
What can I do to make the photos expand properly?
I've got Photos 7.0 and OS 12.3.1 on an M1 MacBook Air.
MacBook Air (2020 or later)