One more question - when you are trying to export the photos, what are you using as the file naming preset?
Photos has a bug, that photos you are exporting with "File naming: Filename: Use Title" will be exported as invisible files, if the title is an empty string. This will create files with a name starting with ".", and such files are invisible. The title will be empty, if you erased a title.
This bug has persisted since many versions of Photos and not yet been fixed. To check, if there are invisible files in the folder you exported your files to, open this folder in the Finder, then enter the keyboard shortcut Shidt-command-period (⇧⌘.). This will make invisible files visible. To hide them again, enter the same shortcut.