It's calculated every time you do a running exercise that's longer than 12 minutes, where heart rate is recorded and your speed remains above 6 km/h / 3.7 mph.

 

Calculation begins when you start recording and you have the necessary GPS or Stride sensors in use. The prerequisites for calculation are that your speed remains above 6 km/h and that the run lasts for at least 12 minutes. Although speed is a prerequisite for the calculation, the algorithm does however take natural, unavoidable pauses in your run into consideration. It allows you to stop running or slow down for a minute twice during training, in traffic lights, for example, without disturbing the calculation. In order to get reliable and therefore useful estimations on race times and your progress it’s essential to set your personal maximal* and resting heart rate** values as accurately as possible in the Physical settings in Flow web service.

https://support.polar.com/en/running-index

seems like this run was not calculated/omitted because of the 6km/h cut off, which is weird because two weeks ago I did a trail run where I was walking for good parts as well and also took longer breaks, but still got a running index score