Screen doesn't shutdown

Hi,
I’m on a fresh installation of Silverblue 39 and I’ve setup my laptop to not go to sleep but just shutdown screens (integrated and external) after 10’ if plugged in, but after the time passed the screens just go black but still on and you can also see the mouse pointer.
I’ve only layered proprietary Nvidia drivers to make the external monitor work.
I’ve search around and seems to be a common problem, but I’ve not found any solution.

Can someone help me?

Thanks in advance