Some of our recent updates have included changes to how the GUI is rendered. I’ve checked with the team since then and they’re not able to currently reproduce the issue that you’re seeing. We’ve tried with a number of setups, GPUs, driver versions, etc, and it is still not occurring for us.
Can you try uninstalling / reinstalling the drivers? Are you using any software to manage your monitors that might not be common / something that we haven’t tested with?
I tried upgrading to the latest nvidia drivers (451.48) and this fixed the issue since the application window will now always open on the primary display when starting Roon.