I think that you are looking for the wrong thing in your original URL - OnStreetBusCoachStopPair is a ParentId for a group (typically two) of bus stops on opposite sides of the road.The actual bus stops are the siblings. Broadly 490Gxxx is a parent and 4900xxx is an actual bus stop
I am wondering whether you have just been unlucky with the choice of stop. 490000062C is the first stop at Ealing Broadway Station on the 65. It so happens that the stops used at Ealing Broadway are about to change and it may be that preparatory work has taken that stop out of service. Its SMS code is 50606 and I also see no arrivals (on London Vehicle Finder). I do however see arrivals at the next stop which has SMS code 58330 and Naptan/Atco code 490006291L. Do you still get an empty array if you try this code?
There is a broader issue here - should it not be possible to retrieve arrivals at ALL stops within the parent rather than zero on on one particular stop? If I want to go from Enfield Town to near Highlands Village I can do it by 456 or W9 but they serve different stops in Enfield Town, just tens of metres apart on the same side of Cecil Road. There are three different stops (probably not with the same parent, admittedly) offering services to Chase Farm, two to Brimsdown, two to Palmers Green, three to Edmonton Green…
I did manage to get it working, I think it was a joint issue of the stops I was picking to be closed and also me using the pair cluster naptanId as you said! As soon as I used the stop you mentioned and used the child Id instead of the parent one it worked!
There may be some data mismatches between our API and the source system (Countdown) that would cause this behaviour. We’re looking into getting data aligned properly at the moment, so hopefully that would minimise these cases.