What about multiple backups? iCloud backup cares only about the latest backup. iTunes on the other hand, can allow multiple backups. Those backups can also be carried around easily. Yes, it should “just work”, but the era of Apple services and devices “just work” is long gone.my iCloud backups have worked flawlessly for years. maybe try signing out and back in. Or take it to the Apple Store for them to check out. That is not normal behavior and should backup overnight while you are charging it.