iBooks App Not in iCloud Backup List
I've just noticed that on both my iPhone 6 and my iPad Air 2 if I go to Settings/iCloud/Storage/Manage Storage/BACKUPS/<this device>/BACKUP OPTIONS, where it lists all the apps and how much storage they occupy in iCloud backups (and allows you to switch individual app backups on or off), the iBooks app is not listed at all. Is this expected behaviour?
My iBooks app has several GB of books stored in it, it is one of the top storage users on both my iPad and iPhone. Most are non-DRM epubs amd PDFs I sideloaded via iTunes.I know that sideloaded epub books and PDFs in iBooks do get backed up to iCloud, but why is the iBooks app not listed, why no option to include/exclude it from iCloud backups like every other iOS app?
iPad Air 2 Wi-Fi, iOS 8.4, 128GB, WiFi