HRM Pro not displaying time in heart rate zones incorrectly

Hi Folks,

During a martial arts class my HRM Pro plus is not correctly displaying my time in heart rate zones.

For example during a 2 plus hours class the top graph seems to be showing the full class but the bottom does not like I have completed a much shorter class

Using my vivoactive 5 watch to start and stop my workouts with the watch.

 Does anyone know what is happening here?

Thanks in advance!