I just had a look at the formula and it will return “null” (cannot be calculated) if there is no time in S3 or time in S3 > time in S1. This is using the 3 zone model (S1 = Z1+Z2, S2 = Z3+Z4, S3 = Z5+). As you can see that is why your 17th September race has no PI:
As to why it is like this you need to have a look at the paper:
The PI for your training zone distribution for each week is shown on the calendar and in the popup when you click that:
You can see PI for any time period on the /totals page.