I have a Raspberry Pi 4 that by default installation of Ubuntu 18.04, will not boot if the HDMI is disconnected or the display is turned off.

Raspberry Pi OS seems to have the same bug

https://www.raspberrypi.org/forums/v....php?p=1547389

On the page, a solution was posted by davidcotot "Adding hdmi_force_hotplug=1 to /boot/config.txt seems to have solved the problem. The Pi4 is running headless,". However this is specific to Raspberry Pi OS.

I found a related article:

https://askubuntu.com/questions/1274...ble-on-a-rpi3b

where a user added the lines to /boot/firmware/usercfg.txt

hdmi_force_hotplug=1
hdmi_group=1
hdmi_mode=16

and on a Raspberry Pi 4, this appears to correct the problem. The Pi 4 booted at least twice with the HDMI cable unplugged during power-on.