Black screen and not responding when starting Retrobat on second display

Hello,

Thanks for all your work on Retrobat - I love it!

I'm trying to run Retrobat on my second display - a TV connected via HDMI. With monitor index set to 1 in BatGui, EmulationStation opens on the display but 'not responding' and black screen.

But if I set monitor index to 0, start Retrobat on my main display and drag the window to the second display - it's absolutely fine. But this is only doable when not full screen.

I've seen other mentions of black screens with HDMI second screens, and I've tried setting the scaling and display resolution on both monitors the same. Also updated to latest Nvidia drivers.

Is there a log I could check to shed more light on this?

Retrobat 6.4.0 and Windows 11

Thanks!