Right now on my system all I have is an Interior Motion Detector watching a temporarily vacant house. Anytime someone re-arms it from the panel in away mode, it changes to stay mode. Which I assume, would not alarm on an internal motion sensor. I am able to disarm and re-arm to away remotely, but I am wondering what could cause this problem.
My best guess is that your Q26 is enabled. This feature determines if the system automatically switches to stay arming (home) even if it was armed in the away mode if the exit/entry delay is never violated this doesn’t including arming in the away mode done with a key fob.
If it is, changing it to (0) disabled will allow you to arm away without an additional sensor acting an exit/entry point to verify you have in fact left the building.
Important to keep in mind is that, although you will still have an exit delay, you will have to remember to disarm remotely before you enter else the alarm will be activated once the motion detector is activated.