@abudef
I ran into the same thing on a dell latitude 5411, but I eventually found a workaround. In the BIOS under System Configuration there was an option called Unobtrusive Mode, and when enabled, a specific key combination will turn off the display and speakers, independent of the OS. The same key combination will then turn them back on when needed.

While an automatic feature would be preferable, the manual option meets my needs since it would be rare to need to use the laptop screen. The one downside for me is that if power is lost long enough that the battery drains, the screen will need manual intervention after power is restored - the BIOS will turn the machine back on when it receives power, but won't put the screen back in its previous state.

I put a sticker on the laptop with the key combination so I will remember it when needed.