Because PrimaryKeyFields is a property which is only valid for when the fields object is in an entity. We refactored the entity fields object in such a way that values aren't stored inside the field objects themselves anymore.
We didn't anticipate that outside entities this property would be used (as it has no designed value outside entities). To mimic the property, do:
var allPkFields = typedList.GetFieldsInfo().Where(f=>f.IsPrimaryKey).ToList();