Ah, hold up a minute. So am I right in assuming you're using the F5 and F6 keys by holding them down while in the different ranges, without letting them go when the power lever reaches it's limits?
If you press F5, while in the high idle range, to bring the power lever to the bottom of that range, but you keep the button pressed AFTER it reaches the high idle position, X-Plane will read that value as zero, instead of the mid point. So when you drag it over to low idle, because of the value X-Plane has recorded (zero), it will snap the power lever to cutoff. If you were to use hardware, and you brought the power lever back to high idle, you would need to bring the hardware throttle to about half way only. Bringing it back all the way will have the same effect if you were to then move the power lever to low idle. It would snap to cutoff.
So, in case I don't make any sense, if you're at full power, press the F5 key until you reach high idle. Then STOP. Don't keep the button pressed. Then drag the power lever over. It should stay at low idle. This will also work in reverse. Moving from START to low idle by pressing F6, do not keep it pressed longer than it takes for the power lever to reach low idle.
Hope this makes sense.