In case this helps anyone else who sees the same thing - after a few months of operating happily, my indoor sensor lost its config (SSID, settings, etc) yesterday evening. Another device that’s been operating for a year, a wifi/HomeKit-connected “smart” power plug, also lost its entire config.
I’m in Seattle and today was the 2nd day of a very intensive “Coronal mass ejection” event (https://www.seattletimes.com/seattle-news/science/strong-solar-storm-could-disrupt-communications-and-produce-northern-lights-in-us/). Obviously it could be a coincidence, but I’ve never had a smart device lose its config, let alone two in one night. I don’t think the devices could do anything to prevent this. The house didn’t lose power and, as far as I can tell, no other devices are affected.
So, if anyone else wakes up to a notification that their sensor is offline and a sensor that’s showing its default “XXs to connect to wifi” pre-setup screen, this could be why 