Since macOS 10.15 Catalina the Photos.app is renaming our files on import with long, unique filenames. See: How Photos 5.0 on Catalina Manages original Files and Filenames
In the Info we are still seeing the original filenames, and when we export images from Photos the original filenames will be restored, but internally the files are stored with their new names.
When we sync images manually to the iPhone or iPad using the Finder, Photos is creating new, optimized versions for syncing. I have not tested this on iOS 18 and Sequoia, but Photos seems now to be using the new, unique internal names for the synced versions. It would not help to switch from the manual syncing to iCloud Photos. I cannot search for filenames in Photos iOS either since upgrading to iOS 18. I can only find photos by the filename, when I copy the filename to the caption (when syncing with iCloud Photos).
To fix this, you could export the photos you want to sync to your devices from Photos to a a folder in your Pictures folder, using the original filenames as the filename and then sync them from this folder. Are you then seeing the original filenames on your iPad/iPhone in Photos?
