Transferring save data backward from the PS5 version to the PS4 version is not supported.
Creating manual backups is the safest way to protect your progress: Dynasty Warriors 9 Empires Save Data
Windows OneDrive can interfere with game saves stored in the Documents folder. To resolve: Transferring save data backward from the PS5 version
Check your PlayStation Plus Cloud Storage or Xbox Cloud Saves. Download a previous cloud save that predates the corruption. Dynasty Warriors 9 Empires Save Data