I signed out and then back into iCloud and lost half my photo library (thousands of photos/videos). Does anyone know what causes this and if it is fixable?

I worked with a tech to fix a bug in my iPhone 11. I constantly have iCloud syncing and am always signed in. My photos are always synced with iCloud for this reason. Prior to signing in and out of iCloud, my camera roll Recents contained almost 9,000 images and video. (Definitely excessive and full of junk... I know.) After singing back in, however, it now has just over 4,000. I’ve been playing around and reading other forums and can’t figure out what’s up. Can someone point me in the direction of a fix, or possibly explain why if it isn’t fixable? Is it possible that it’s simply taking hours to retrieve the library in full and I’m just being impatient? Thanks!