Individual PDF books getting password locked and inaccessible on iPad and cloud?

I've had a sporadic issue where PDFs I've bought through DriveThruRPG, then uploaded into Books on my iMac and then synched with my iPad will suddenly ask for a password, books I've had and read for years. It does not accept my Apple password and I have to delete the book and re-upload it to get access again. There's no rhyme or reason to it and it's infrequent.

When I upgraded to a 2023 iMac last week, it uploaded all of the contents of my Books to the cloud, against my wishes and without my consent and made my synched books inaccessible apart from iCloud.

Today, the password issue happened again on the pad. I'd been reading the book in the morning, when I went back in the evening, it was password locked.

When I got home and went to iCloud on the Mac, it shows it likewise password locked and not accepting my password. The cloud icon is different too, a dotted line instead of a solid one. EDIT: It is not accessible through Books either, the same lock icon over it.

I called support about it a while back, when it was just on the pad, not the cloud, and they couldn't figure out what to do,