The Android device being charged is not displayed at all and is dark … This is normal now, but it may change in the future. This is because “Ambient Mode” is added to Android OS.
Ambient mode is a display mode dedicated to charging that is added to the Android OS. In addition to displaying information such as the current time, weather forecast, and calendar schedule on the display, you can use Google Assistant functions. A small display device with voice AI function is called "smart display", but ambient mode may be easy to understand if you think of a function that makes a charged Android device smart display.
Ambient mode also has the ability to continue to display photo slideshows. Since Android devices can be used like a digital photo frame, this is more suitable for tablets than smartphones with smaller screens.
Ambient mode is not provided on all Android devices. It runs on Android 8.1 or later, but as of December 1, 2019, the supported devices are limited to SONY, Nokia, Transsion (conducting technology), Xiaomi (small rice) smartphones, and some Lenovo tablet products.
Some Android devices have a function called “Always-on display”, which is similar to the ambient mode. Ambient mode works when charging regardless of the display method, but ambient display works for devices that use an organic EL panel and works when sleeping or in hand.