Issue booting headless FreeBSD system, requires monitor to boot

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. It boots as expected if I plug in an HDMI cable to a monitor before powering up the machine. 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.