Screen doesn't turn off when lid closed and external power connected

2019 MBP running Big Sur 11.6.1 fully patched this week

Reading this post https://discussions.apple.com/thread/252206907 a person with a similar problem was advised to unplug the computer to make the screen turn of when the lid is closed.

In my case this is not a legitimate work around. I have an external monitor, keyboard and trackpad connected and I want the internal screen to disable when the lid is closed and the power is connected. If the internal monitor doesn't disable then any windows on that screen stay on that screen and my mouse often gets lost over there.

I've verified that the lid angle sensor functions properly when the power is disconnected, so evidence points at this being a software issue. Is there a setting I haven't been able to find that controls this behavior?