I recall some other reports of issues with dual-screen configurations that were difficult-to-impossible to replicated and track down.
One thing that caught my eye was that some of the problems occurred only when the rightmost monitor was the main screen (ie, carried the menu bar). That means the aux monitor is using negative screen coordinates, which may or may not be an issue worth looking at.
Secondary screen is right of main screen. It looks like screen coordinates are clipped to the main screen.
When moving an item with hint text to the extreme right of the main screen, you'll notice the hint falling of the screen. This is what the displayed hint looks like when triggered from the secondary monitor.
So what Stefan suggests is not the case as the part 'falling of the screen' appears on the second monitor anyway.