Stop custom function from re-running

I successfully followed this Microsoft guide:  How to extract key phrases in Text Analytics

Next day, I opened Query Editor to add an Index column.  When I applied the change, it appears that the custom function to extract key phrases re-runs. I get this impression because Power BI is connecting to the API endpoint.

Re-running the custom function in not necessary - no new text was added.  All it does is increase the time it takes to add a simple index column.  Worse though is that it adds to my API usage. 

Is my understanding as set out above correct?

If yes, how do I prevent the custom function from re-running?

Ideally, I only want the custom function to run on new data that I add to the data set.  Is this possible?

If my understanding is incorrect, then why does Power BI connect to the API endpoint?

Thanks

Duffy