Instinct 2 solar pool swimming time in heart rate zones are not right

Hi, Just got my new watch instinct 2 solar and had the first swim in the pool with it. I have found out that the values of time in heart rate zones are not right. For a workout of 1,5 hour I have got a value of 10h in zone 1 for example and 5+ hours in zone 2, etc.


any ideas why it is happening? Is it a bug in software on the watch ?