Raspberry PI Monitor 15.6" not starting with RiscOS

I have difficulties getting my Raspberry PI 400 to start with the new official Raspberry Pi monitor (15.6"). If I power the monitor via USB port directly from the Raspberry Pi, the monitor is not detected during start up and goes to sleep. Turning it back on or plugging it back in doesn’t work because the Raspberry PI didn’t recognise the monitor during the boot procedure and is sending the wrong HDMI signal.
I have tried hardcoding the HDMI group and mode (82) in the CONFIG.TXT file but that didn’t make any difference.

There is a workaround though involving a little bit of effort: Power the Raspberry Pi monitor via an external USB-C power supply initially. Disconnect the Raspberry Pi computer from its own power supply, then reboot it. In this way, it will recognise the monitor, which can then be changed back from using an external power supply to the Pi’s USB port for power (while keeping the Pi running, of course).

Does it work if you add:

hdmi_force_hotplug=1

to the config/txt file in !Boot.Loader?

Thanks for the tip, that worked – as long as the Raspberry Pi Monitor is plugged into the correct HDMI port (the Raspberry Pi 400 has two HDMI ports), the Pi recognises the monitor and sets the resolution to a beautiful 1920 × 1080 resolution. The monitor doesn’t come on, if it is plugged into the other HDMI port. Also, the official Pi monitor wants the disable_overscan=1 statement in the CONFIG/TXT file, otherwise there is a big black frame around the picture.