iPhone 12 Pro Overnight battery drain No Cell Coverage

I usually plop my iPhone 12 Pro (running iOS 14.6) on it's wireless charger when I go to bed. Last night, I noticed it said it was 100% charged, so I didn't bother. Just picked it up, and it's dead! Plugged into a wired charger, and when it booted up, I went to check Settings / Battery to find out what caused it. Battery says that "No Cell Coverage" used more than 80%!! Say what? I'm in the middle of a large city with fabulous cell phone coverage. Yes, I turned off 5G because that was really draining the battery, and I don't need it. What in the world is the phone doing that it thinks it has to keep looking for cell coverage overnight?

One more detail: I use dual SIMs - the eSIM is for my main provider at home and the physical SIM is for the carrier I use when I travel abroad (but also works on roaming when I'm at home). I usually leave both on because I often get important text messages on the "abroad" SIM. I'll try turning that off tonight to see if that makes a difference.