I am a novice to that tbh. I was exploring the use of ref:STIB_MIVB to "group" the new (beta) real time api of waiting time on both platform  to link it on the main station for a personal iot application. (but i can use another way to do that so it's not a blocking problem)

If you look on https://gtfs.org/fr/documentation/schedule/schedule-best-practices/# it's say that "Le champ parent_station doit faire référence au stop_id de la gare dans laquelle se trouve l’établissement d’embarquement." and for this station it's 5. all platform here or entrance are linked to 5  not to 27 

If you take https://www.openstreetmap.org/node/6414192032 it's ref:STIB_MIVB 	67 and that's match the gtfs data

If i follow the logic you explain https://www.openstreetmap.org/way/684571186 should be linked to station 70 (diamant)