I still have an old GPU in my PC due to the shortages and overpricing. I just got a new PG329Q monitor and connected it via displayport and when I turn the PC on I get one long beep and three short beeps which means 'No VGA' but the PC does still get to windows. My guess is this is due to the GPU not being fully compatible with displayport despite having the socket.
Since my earlier post I've found that if I turn on the screen and immediately turn on the PC the BIOS will complain but if I wait for the screen to show 'Displayport no signal' and then turn on the PC there will be no issue and the BIOS splash screen will show and go straight into windows.
I got around to getting the HDMI cable out of the loft and using that instead of DP and the beeps are gone and no downsides that I've noticed. Running at 144hz. Not sure if variable refresh is working as it should but I wasn't sure if that was working with my old GPU anyway.