Thanks James
I think I have now sussed why the road dropped out of CS. The stretch that I ran is labelled as trunk and I can see that in the overpass query this is now excluded. I know there was a debate on this previously as in UK a lot of roads are correctly labelled as trunk but are runnable The Trouble with Trunk Roads.
At the same time I can see that the boundaries were changes in OSM 9 months ago which would shift this specific road into a different sub-city (if it were included).
Agree a definite edge case and was very much a curiosity rather than a city I care about for my current stats!