new iOS Books app still deletes ebooks from internal storage at random

[shiphawk]

Today, I discovered that the new iOS books app (used to be iBooks) has deleted all of my ebooks from my iPhone's internal storage. I can re-download them via iCloud but, of course, this requires an internet connection which I was without on a long flight today. This bug has persisted for years but I foolishly assumed that Apple had addressed the issue when they rewrote the app for iOS 12. Has anyone come up with a solution for this? The issue has been posted to Apple's support forum but has gone unaddressed for years. I'm now wondering if they designed the app to "offload" ebooks under certain unknown circumstances. I cannot find a setting to control that however.