I know this thread is closed but just adding another data point; I just saw the same as the OP. I installed brand new mailbox sensor on 2021/10/18, then on 2021/12/24 it disappeared from my ring.com. I found all AAA batteries were very low and replacing them with brand new AAAs got the device back online.
I’m in Atlanta, GA. Indeed the temp went down to 0C in the morning for the last 1 month before it went offline.
Oh, and I wonder if I could’ve changed them without removing the mounting tape?
This, I was able to replace batteries without breaking any tape (I bought the sensor in question in 2021/09 FYI).
And no notification for low battery is bad. Reporting in https://community.ring.com/t/centralized-battery-status-check/14240