When all else fails, try rebooting.
Is there a setting in the XML for the wait time before the ss kicks in? I don't see one, so I assume the wait time comes from the Windows ss setting. I set the Windows wait time for 1 minute, and I think that's why the Windows ss kicks in first. That responds to mouse action and goes away, leaving the MB saver showing. But as before, it doesn't close. Probably because, as you say, it doesn't have focus.
To summarize, if I invoke the MB ss from the View menu, monitor 1 has the ss, monitor 2 is active. But I can't clear the ss from monitor 1. If I let the ss wait for 1 minute, the Windows saver shows up on both monitors until a mouse/keyboard event. Then the MB ss shows up on monitor 1 and monitor 2 is active. And again, the ss won't clear unless I get Task Manager. I guess I'm confirming what I said originally.