When you have a nested object where there are 2 properties such that the name of the first property starts with the name of the second property.

public class SubscriberSource
{
/// ...
public DateTime? StatusChangeDate
{ get; set; }
public SubscriberStatus Status { get; set; }
}

And you try to build a projection, it will not generate anything for the projection at all. There is probably somewhere in the code that a .startsWith is utilized to add fields to the projection builder for some reason.
So when i write something like

_collection.Find(query).Project(x => new
{ x.StatusChangeDate, x.Status }
)

No projection is passed to mongoDb at all and all fields are returned.