'Secured with Dell BIOS' persists on switched-off screen

I use an external monitor, and display only on that external monitor when I am at my desk. The "Secured with Dell BIOS" message persists on the laptop screen (which is meant to be inactive) after startup. To be clear, I am fine with the message occurring during startup, there's just no reason for it to burn its message into the otherwise inactive screen. The only workaround I have found, which is clumsy and has to be repeated after every reboot, is to go into Settings>Display, change the monitor selection, then revert to displaying only on the external monitor. 

This is not the same problem as reported here, which is an aesthetic complaint about having to see the message at all

https://www.dell.com/community/en/conversations/xps/xps-13-plus-9320-secured-with-dell-safebios/658de4ae9222566d9dac57b1?page=2