This is now a long-standing issue: it really makes it very frustrating working with Cubase these days. Mixer or Arrange windows just end up back on the main monitor, even though saved positioned on a second screen. They can be coaxed back, but next time Cubase is launched, it’s all forgotten and everything piles up on the main monitor again. This was not the case in Cubase 6.
When it actually (kinda) works, when switching workspaces windows saved on the second monitor strangely move toward the first monitor and back again, instead of staying put. Channel settings panes stretch to full width and then don’t resize properly, content disappearing outside the pane. It’s all very clunky. And been this way through all revisions of v8. Please fix!