https://www.techradar.com/news/iphone-xs-charging-fix-now-available-in-new-ios-1201-update

Last week, we reported on an iPhone XS and iPhone XS Max charging bug that saw some handsets fail to receive power if plugged in while the display was asleep. 

In almost every reported instance, the phone would begin to charge normally once it had been unlocked and interacted with, indicating that the issue lay in the device's inactivity, and not the display's sleep status.