Can not boot without monitor attached

Hi,

I'm trying to set up a headless FreeBSD 13.1 system.  I've done so
successfully before on this particular machine, but for whatever reason,
after a complete reinstall, it doesn't want to boot without a monitor
attached via HDMI.  Since it never shows up on the network I'm assuming
it never even gets past the boot prompt.

If I plug in a HDMI cable to a monitor before booting, it boots as
expected.  Plugging in a monitor after unsuccessfully trying to boot
without a monitor does not work (black display).

I tried adding -P to /boot.config, but this had no effect.  The system
is, apart from that, fairly vanilla.

The system is a fan-less Chinese(?) "Basoaro" system with an American
Megatrends BIOS, but I can't find anything in the BIOS that seems
relevant, and since it *used* to be possible to boot the system without
an attached monitor, I'm assuming it's not a BIOS-related issue.

Whatever I did before to get it to boot headless, I've forgotten what it
was and need a reminder and/or pointers from you guys.

Regards,

-- 
Andreas (Kusalananda) Kähäri
SciLifeLab, NBIS, ICM
Uppsala University, Sweden

.