When you get an entity object, by default, only the primitive fields and any to one relations are included in the fields returned. If you want to get the collection fields (to many relations) you have to query specifically for them.

I suppose because there has to be a limit drawn somewhere as to what gets returned when you query an entity, to avoid the query taking too long

Nguồn:: How to get the list of entities in an relation field? - Get Help - Fibery Community