Dell Latitude 5431 + Dell P3223DE -> external LCD is not able to go to stand-by

Hi, I have problem with my laptop Latitude 5431 and LCD Dell P3223DE connected through USB-C power delivery.

LCD monitor is not able to go standby mode and there is still backlight turned ON, but Laptop's LCD is OFF.

How to reproduce:

Scenario 1:

Laptop with connected P3223DE is locked by inactivity, Laptop's LCD is turned OFF, P3223 is turned OFF too but in 2 seconds is turned ON with blackscreen and backlight enabled.

Scenario2:

FN+F8 and ONLY laptop's LCD -> then P3223DE is with blackscreen and backlight enabled too.

Any idea what configure? I updated through Dell Command app my drivers. I tried it update directly through Intel's app too.