If Keyboard Maestro, without any extra permissions (which it does not have), simulating keystrokes on one account, is typing keys into another account, then that is a significant security violation which you should report to Apple.
Keyboard Maestro does not run with any extra permissions, does not install any privileged helper tools (except temporarily for upgrades), so it should not be able to access any other account, or influence it's behaviour in any way. Keyboard Maestro simply adds keys to the event queue.
Either something in your system is passing the keys through from one account to the other, or there is something very broken in the system handling of the event queue.
But there is nothing Keyboard Maestro can do differently in this regard - what you show should be impossible.
Unless you have some hardware or software installed that is intercepting and redirecting the event queue keys, I have no idea what might be going on.