When laptop is docked into docking station, the external monitor never goes to sleep

[Mauro Mozzarelli]

Environment:

Lenovo W530 laptop docked into Lenovo docking station with external monitor, keyboard and mouse.
The Laptop lid is closed.
Ubuntu Mate 20.04 LTS

The system is configured to blank the screen and put the screen to sleep after 10 minutes idle.

The screen is blanked but the monitor is never put to sleep thus remaining active permanently.

This does not happen when the laptop is undocked and the external monitor is connected directly to the laptop as a secondary monitor. In this case when the computer reaches the idle timeout both internal and external monitors are put into sleep mode.

This also did not happen with Ubuntu 18.04 LTS in which case the external monitor was handled correctly also with the laptop docked.

[Mauro Mozzarelli (ezplanet1)]

If I keep the laptop lid open (whilst docked) the external monitor goes to sleep normally when idle.