Data Flow External Call transform does not appear to add row-specific URL, query parameters, or even make call in the first place

Question

I am attempting to make calls to the Google Maps API as part of an ADF data flow.

I initially set up the linked service as follows:

I then set up the data flow as follows, where "key" is the API key, "address" is the address to lookup, and "rowURL" is these two combined in the form of a query string:

When debugging the data flow, the status column showed 400. When Google Maps API returns a 400, it also includes an error_message in the response object, but ADF did not expose the response object, so I could not see it.

In order to see what it was actually sending, I changed the endpoint to a debug endpoint that I control, using PipeDream's ResponseBin - this will show me all requests made against a URL. It showed one request when I used the "Test Connection" button in the editor, but when actually running, it showed 0 (zero) requests being made, rather than the 100+ that were expected. The status column in the data flow showed 415, which was clearly not being sent by the debug endpoint, which is hard-set to return 200 and a small piece of JSON.

It would appear that the External Call transform is not actually hitting the endpoint I think it's hitting, but it also does not appear to expose what it actually did so I have no way of knowing what the actual problem is.