Current charge level is displayed in lieu of the date whenever the iPhone is not actively charging. When charging via Lightning cable, battery case, or wireless charger, the behavior is the same as it is on iOS 12.1.3. The charge level is displayed for a few moments before it swaps over to the date.
It's not known if displaying the charge permanently is the intended behavior, and this is potentially a bug because it's not likely most users want to display battery life continuously in place of the date. We're also seeing some bugginess with the way charge levels and date listings swap on various devices like wireless chargers and Lightning cables.
Apple may be intending for the non-charging behavior to work like the charging behavior, with the iPhone displaying the charge momentarily when woken up before swapping over to the date. If this is a bug, it should be cleared up with the next beta. If Apple does indeed intend to display the battery percentage on the lock screen temporarily in all conditions, it would be a welcome change. Some users have been unhappy with the battery percentage being hidden in the Control Center in the iPhone X and later.
Update: Apple's release notes clarify that this is a bug that is listed in the developer beta release notes. It will be fixed in a future beta.