Usando Dapper per mappare su oggetti nidificati che non funzionano

Il problema

In poche parole, la mappatura non si comporta come previsto. La reason variabile locale torna come nulla, quindi l'assegnazione a nav imposta anche l'oggetto nidificato Reason .

Mi sto perdendo qualcosa di semplice qui?

La stored procedure

ALTER PROCEDURE [PA].[spGetDocumentNavById]
-- Add the parameters for the stored procedure here
@DocumentNavID INT
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;

-- Insert statements for procedure here
SELECT
    dn.DocumentNavID,
    dn.IssuerName,
    nt.Name,
    dn.Amount,
    dn.BTF,
    dn.NavDate,
    dn.ReceivedDate,
    dn.PrimaryIdentifier,
    dn.AssetID,
    dh.LastModifiedBy,
    dh.IsApproved,
    dh.Corrected,
    dh.ReasonID
FROM PA.DocumentNavs dn WITH(NOLOCK)
    JOIN PA.NAVTypes nt WITH(NOLOCK)
        ON nt.NAVTypeID = dn.NavTypeID
    LEFT JOIN PA.DocumentsHistory dh WITH(NOLOCK)
        ON dh.DocumentID = dn.DocumentID
    LEFT JOIN PA.Analysts a WITH(NOLOCK)
        ON a.AnalystID = dh.LastModifiedBy
WHERE dn.DocumentNavID = @DocumentNavID

Il C #

public DocumentNav FindById(int documentNavId)
    {
        var data = new
        {
            documentNavId
        };

        using (var conn = SqlConnection)
        {
            var response = conn.Query<DocumentNav, Reason, DocumentNav>("[PA].spGetDocumentNavById",
                map: (nav, reason) =>
                {
                    nav.Reason = reason;
                    return nav;
                }, param: data, splitOn: "ReasonID", commandType: System.Data.CommandType.StoredProcedure)
                .DefaultIfEmpty().FirstOrDefault();

            return response;
        }
    }