Do you have Google home speakers? i'm having a similar issue where I occasionally wake up around 3-4 am to find my thermostat set to 50 degrees. Whenever this happens I find one of my Google home speakers is offline and unable to reconnect to wifi. I have no idea why it's offline since nothing else disconnected from wifi. It's always a different speaker and it never reconnects until I unplug and plug back in.
I've disabled scheduling, nest sense, Eco mode is nowhere near 50. I have no ides what the cause is.