Interesting! I guess that I'm not using the lowest power mode[1], but for months now I have had a ATMega328P (running on 3-AA batteries) monitoring doors w/reed switches. I haven't had any trouble w/interrupts not firing on change.
1: I'm using Rocket Scream's LowPower.PowerDown for what it's worth.
1: I'm using Rocket Scream's LowPower.PowerDown for what it's worth.