RPM Sensor gets disabled when I press the safety button

I have a Pixhawk 2.1 that I am using with a spektrum rpm sensor for gas engines.
I have had limited success getting it to work. Most of the time if it doesn’t work when I start the engine I just reboot the pixhawk and try again with limited success.
I decided to look into the issue today and found that when I press the safety button to turn on my servos the pixhawk sets the RPM_PIN parameter to -1, although it doesn’t always show up when I read the parameters I can see on my oscilloscope that it pulls AUX pin 2 (The pin I have the rpm sensor connected to) to a low state and it does not reset until I set the RPM_PIN parameter back to 51.
This appears to be a software issue although maybe the spektrum rpm sensor is doing something weird that causes the pixhawk to disable it. Any incite would be greatly appreciated.