I’ve seen this happen frequently in both iBooks and Apple Music, which is why I refuse to use either. I always assumed it was a DRM thing? Kindle doesn’t do this at all, and while Spotify does, it’s much rarer, and always waits until you have connectivity, so it can immediately re-download.

It’s such an obvious deal-breaker, I don’t understand how “let’s delete your stuff all the time, even in offline mode” made it through testing.