Why does my Displayport monitor not turn on when selecting a boot drive?

I'm on OSX 14.5, with a MacPro 2013. I have an ASUS Displayport monitor, and ordinarily it works fine, both in OSX and from Windows 10 on a Bootcamp partition.

But if I boot the Mac holding Option to open the boot drive selection screen, the monitor does not turn on.

How can I get this to work?

EDIT: Extra data... if Win 10 sleeps, the monitor will not wake up.

I was not able to solve the problem with that connection. However, it works fine with an HDMI to HDMI cable.

Leaving the question unanswered, as I'd still like to know why the Thunderbolt to Displayport connection didn't work properly.