Zwift shows different heartrate than on my zwift on my iwatch

Hi at all and happy new year!
I’m using Zwift with my iPhone and my paired iWatch.
When I’m in Zwift it shows an heartrate of maybe 80 or 100 bpm and also changes, but on my Zwift iWatch there is shown the real heartrate of 140 and also changes. This will last for the whole race an then my average over the race is wrong. Everytime I look at the Zwift App on my iWatch the heartrate is shown correct. I did some races where everything was okay.

Has anybody same issues or an idea?