Activity app and Health app give false HR data from Series 3 Watch

In my last two swimming workouts with my Series 3 Watch, my heart rate data in the activity app says my average HR was 156 bpm and, not recorded, respectively. Looking at the data on the Health app on my iPhone shows readings that are consistent with the activity app. These rates are wrong because (1) I am 76 years old and my max HR is about 155 bpm and (2) I get correct readings using my Polar A300 watch which is much more accurate than the Apple Watch. I have used both together previously and got essentially the same readings on both.

Last week I updated my Apple Watch to IOS 4.3.2 and then the problem began. Anybody else have this problem? It is not due to interference between the Apple watch and the Polar A300 because today I wore only my Apple Watch.