Apple Watch Activity session show different heart rate than Garmin HRM Pro

Hi, I'm using the Garmin HRM Pro as external heart rate monitor connecting it via bluetooth to the Apple Watch. Today I recorded a workout session with Apple Activity App wearing the HRM and when I finished I saw different values, Apple Activity App shows 176 max heart rate and Garmin HRM shows 164 as max value during the same session. Why? With the external HRM connect Apple Watch should stop using his sensor recording the heart rate from chest band so I expect the same value. What's the point of using a chest band if its values aren't recorded?

Is present some bug and AW lose connection with HRM and came back to record with his sensor?

I'm using WatchOS 7.4 (18T5169f)

Thanks, any suggest it will appreciated.