Brand new contact sensor shows 'battery out of power', but it works only in disarmed mode

I'm just installing my first contacts sensor (gen 2). Every time I open the door, I hear a chirp and see a brief green light. I also see in the device history that the door was opened or closed. Everything seems to work as I expect until I try to change to home or away mode. Then the keypad says 'sensors require bypass'. I also see 'battery out of power' for that sensor in my app. I can't believe that the batteries would be low when I just removed the plastic tab, and I can't believe the sensor would work so well if the batteries are out of power. Is there something else that could be causing this?