I use Roon exclusively on my tertiary monitor, but that is not respected the next time Roon is started when it always appear on the main monitor and I need to manually drag it where I left it the last time.
I’ve also noticed that Roon splash screen is not centered well on the monitor, probably due to DPI issues, but that’s not something I really care about.
I believe the first part (opening Roon on the same monitor as the last session) should be done. It’s difficult to tell whether that is a feature request or a bug report.