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GRUB is not started without monitor

I have installed linux server with GRUB. It works fine with monitor connected. But if unplug monitor and reboot server - GRUB is not starting.

I have tried to load other linux (without GRUB) from USB stick - its booting fine without monitor.

I have tried many options (like GRUB_TERMINAL=console) in /etc/default/grub, but it useless because GRUB not starting at all. I have added play command at the top of /boot/grub/grub.cfg file. Tone was played with monitor connected but not played without monitor. So all options in /boot/grub/grub.cfg file is useless because it not loaded.

The last option is to make dummy VGA cable to simulate monitor presence, but I still hope that the problem can solved without any "hardware" :)

What could be the problem? Is there any way to enable GRUB verbose logging on failed boot and read it after successful boot?