Photos disappearing when copied/moved

I have an album on my internal storage that has 2800 photos in it. A few months ago I tried to copy them over to my SD card (to see how much space they took up) but the new album only had 1600 photos. It's not happening with any other album and I'm not sure why. Tried again earlier and it's always the same, it'll select all 2800+ images, shows the bar loading photos through 2800+ and yet it comes out to 1600.

Anything I can do with that? My phone is and has been fully up to date, as are all apps.