Recent IOS updates on iPhone cause Watch to burn through battery

I have an Apple Watch model 1554. If I take it off the charger at 100% and wear it for a couple of hours, then take it off to shower, it will give Low Power warnings when I put it back on. If I just brush my teeth after taking the Watch off and check the battery level from my iPhone, it will have dropped by more than 40%. But if I put it back on the charger while I shower, the battery will last the rest of the day. If I put it on at the start of the day and don't take it off, that battery lasts all day. The watchOS has not been updated recently, only the iPhone.

I've seen reports about battery life issues with Series 3 Watches, but mine is older than that.

This problem started 2 iOS updates ago, and continues with the most recent update.