Cadence sensor disconnects within first mile of ride

I've been using my Edge 25 for about two years, with no problems until March 2021. The magnetless cadence sensor has begun to disconnect within the first mile of a ride, usually around 0.8 miles, and doesn't reconnect until I finish the ride and hit the Stop button. There's no message on the Edge to alert me of the disconnection. I've already tried the obvious remedy of changing the sensor battery. It's worked correctly once in the last five rides.

I've also noticed that my iPhone 8 (iOS 14), with the Garmin Connect app installed but not open, has begun syncing automatically with the Edge. It never used to do that; I had to open the app and initiate the sync. I don't know if these two things are related, or just coincidence.

How do I make the sensor stay connected?