iphone – iBooks automatically removes my books offline and when there is enough free space on the device

There is something odd with iBooks on iPad and iPhone: it removes some books locally and keeps them in the cloud.

It's frustrating because it happens when I'm offline and I simply lose access to them.

Why does this happen at all? And how can I stop it?

I've enabled sync on iCloud Drive and everything else, but I do not see any option to disable this feature. optimizations.

PS: the same thing happens with some pictures on iPad. They show a I sign in the lower right corner, and I can not see them of good quality unless you have access to the Internet.