Missing Registrations on arriving / Departing flts

I’m currently working on something that will query then filter all arrivals / departures for a certain airport. I’m using AeroAPI and

GET /airports/{id}/flights/scheduled_arrivals
GET /airports/{id}/flights/scheduled_departures

I’ve noticed the registration data for the aircraft is patchy, its sometimes there sometimes blank. Yet when I query the flight with no tail no. on the FA homepage all details are there.

Is there a better query to use for this information without querying each flight by number individually?

Could you provide a specific example of where we are showing the registration on our page but not within the API?