To reproduce this you need to position the desktop alert on a monitor different than the primary one. Also, the DPI scale settings on the monitors should be different. For example, the primary one can have 100% and the second one 125%. The "dpiAwareness" setting also affects this behavior.
To work this around, you can host the RadDesktopAlert control in a new Window control instead of using the RadDesktopAlertManager. This will give you full control over the alert window's position. In this case the built-in dragging and auto-closing won't work, so it should be implemented manually.