Disappearing old purchases from Apple Books iCloud

My old purchases keep on disappearing from my Apple Books library.

I have a large books library of books, over 5000 (large amount of manga).

Every time I purchase a new book, old books randomly keep on changing their status from “download icon”to $(price). once I click on the item detail page, it updates the status of that book to “download icon”.

however, entire series disappear. And sometimes I get the Apple Books app randomly removing books in real time.

This means the only way to consistently read older books from my library is to go into purchase history and hope the books exist there. Sometimes books in my library but with a “reset” status will appear in library. But that is not guaranteed at all.

What is with this arbitrary book limit on my iPad? iTunes on the PC has the correct list of purchases, but the Apple Books app is broken