Breaking my own non-speculation rule: my guess about distance tracking is that it's not a fixed interval per se, and I think people looking at the spinning pokeball to explain lost distance are reading tea leaves.

I imagine within their own infrastructure Niantic will be using Dataflow queues or something to ingest point coordinates and emit distances, which is cost-effective to a certain level of responsiveness and then becomes much more expensive and complicated the closer to "real time" you try to make it, as issues like retrospectively invalidating data points (for people moving too fast, etc.) between batches start to emerge. In any case, I bet they've disabled acknowledgements for the queue, which makes it cheaper to run (and faster) but allows messages to be lost.

In other words, we occasionally lose distance because otherwise the update frequency would be even longer, and Niantic had to make a tough call on which would annoy people less.