Books app files automatically deleted during update ipadOS27

Alot of files that i had downloaded and using on ipad 4th gen 64gb just vanished after updating to ipadOS27 . Can they be recovered, i had unsynced books app from icloud due to low icloud storage but these files were supposed to be in my internal storage right? How come they just deleted, i had recently done optimse storage during OS update. Has anyone faced similiar issue?