[atlas] Breaking API change?

ISC's Root System Atlas visualiser used to use this API call to access 
the built-in root system measurements:

<${apiUrl}/measurements/${m}/latest/?fields=responses.0.response_time,responses.0.abuf.answers.0.data.0&freshness=1800>

where ${m} is the measurement number.

It used to return this object:

{
   probe_id1: [ [ latency, site1 ], ... ]
   probe_id2: [ [ latency, site1 ], ... ]
   ...
}

It now returns this array instead:

[
   [probe_id1, latency, site],
   ...
   [probe_id1, latency, site],

   [probe_id2, latency, site],
   ...

]

and I had to explicitly request the probe_id field in order to get it.

Was this change intentional, a regression, or did I not use the API 
right in the first place?

cheers,

Ray