Basically, the XP11 brought multi-window support with it and changed the way that window-coords are shared with plugins.
Gizmo and the Saab were written pre XP11 so they do not account fully for these new coords.
As you've discovered, the monitor that X-Plane resides on matters, it's not so much the physical location of the monitor as it is the logical-id of the monitor.
Eg: Your 16:19 screen is probably logical ID 1 and 21:9 might be 2 or 3.
The fact that screen-1 is in the center doesn't matter. X-Plane will feed us useful position data.
When on screen-2 (far left, 21:9) the position data is somewhere in the negative values and thus the tool-tray mouse detect code doesn't fire correctly.
Better multi-monitor support has been on my list for a long time now but life has a habit of getting in the way.
There is currently a dev-beta of Gizmo64 v20.x being tested privately which has support for the new XP11 pop out windows and so on so support is coming for all the multi-screen goodies but it takes time.
For now, the best you can do is continue to move the sim window around. Sorry I can't give you a better solution.