Hi Russ,
referring specifically to merthyr bus station but this prop applies elsewhere too. When available public_transport=stop_position nodes should also be added to the relation directly before the corresponding platform node with the role of "stop" and the service road leading to the stop position should be added a 2nd time so you have it following the service road into the stop position and then following it again as it reverses out of the stop position and forms a continuous path.
Kind regards,
Kits