Some, but not all, iOS 12 users complain their iPhone or iPad won’t begin charging if it’s in sleep mode when a Lightning cable is plugged in. Apple has introduced a beta of iOS 12.1 that appears to fix this problem.
The charging issue doesn’t occur if the device is awake when the cable is plugged in. It only crops up when a phone’s or tablet’s screen is off.
There have also been reports of a similar problem if the iOS device is asleep when it’s placed on a wireless charger.
As mentioned, this issue doesn’t happen to every computer running iOS 12. But it does seem to affect a wide variety of devices, old and new.
A fix in iOS 12.1 beta 2
Apple just introduced to developers a second beta version of iOS 12.1. According to published reports, this fixes the charging problem. Cult of Mac can’t confirm this because neither our iPhone XS Max or our 12.9-inch iPad Pro ever exhibited the issue.
This leaves those afflicted with this failure-to-wake problem the choice of either installing the public version of iOS 12.1 beta 2 when it’s released sometime soon, or always waking their iPhone/iPad up before setting it to charge.