Drawing area not showing new lineart

I’m working from home on my laptop, using a new Cintiq 22HD. There’s no problem when both screens are showing the same thing (aka duplicate displays), but when I extend my display so I can work in Toon Boom Harmony (15 Advanced, in case that might be relevant) on my Cintiq and open reference images on my laptop screen, things get weird.

I can draw no problem, but I can’t actually see what I’m drawing. It’s like my drawing area doesn’t want to refresh to show what I’m doing. Drawing in an empty drawing area will look like it remains empty. But then when I simply drag the window back to my laptop and maximize it, things work as they should, and the previous lines I couldn’t see are now there. So it seems that everything works fine, it’s just not showing up on my Cintiq.