Date added is stored along with other metadata in MusicBeeLibrary.mbl file in your library folder.
I don't think it's about your Windows but you might have moved those files or changed their filename with MB closed. Hence MB no longer recognize them as existing files.
If you restore their original file URL and open MB again, then it might recognize them again showing their date added values.
for a while i notice that i have to manually add the time and date within mb when files are included.
If you have properly imported your files to either inbox or library, that can't happen. I wonder you ever imported your files to the library.