LinqToSql DataContext with extracted interface doesn't compose SQL for functions

I have setup a test database and console app to confirm the following:

Given a SQL Database with the following function:

CREATE FUNCTION ufn_GTFO
(
    @Guid as uniqueidentifier
)
RETURNS VARCHAR(100)
AS
BEGIN
    -- Declare the return variable here
    DECLARE @Result as VARCHAR(100)

    -- Add the T-SQL statements to compute the return value here
    SELECT @Result = 'This is a test'

    -- Return the result of the function
    RETURN @Result
END
GO

And the following table:

CREATE TABLE [dbo].[Test](
    [PKey] [int] IDENTITY(1,1) NOT NULL,
    [WFT] [uniqueidentifier] NULL,
 CONSTRAINT [PK_Test] PRIMARY KEY CLUSTERED 
(
    [PKey] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

The Scalar valued function ufn_GTFO is normally composable, such that the following C#:

static void ConcreteTest()
{
    DataClasses1DataContext db = new DataClasses1DataContext();
    var q = from t in db.Tests
            select new { t.PKey, GTFO = db.ufn_GTFO(t.WFT) };

    var p = q.ToArray();
}

Is translated into the following SQL:

SELECT [t0].[PKey], [dbo].[ufn_GTFO]([t0].[WFT]) AS [GTFO] FROM [dbo].[Test] AS [t0]

However, if I use the refactor -> extract interface on the DataContext, and use an instance of that:

static void InterfaceTest()
{
    IDataClasses1DataContext db = new DataClasses1DataContext();
    var q = from t in db.Tests
            select new { t.PKey, GTFO = db.ufn_GTFO(t.WFT) };

    var p = q.ToArray();
}

I get the following SQL, and calls to ufn_GTFO occur once for each record as .ToArray() enumerates the results.

SELECT [t0].[PKey], [t0].[WFT] AS [guid]
FROM [dbo].[Test] AS [t0]

So, my question is why does this happen and what can I do to prevent it while still using the interface?

Update 1: I've compared the IL generated for the concrete method versus the interface method, and they differ only in the reference to the interface and a compiler generated display class that doesn't seem to have any bearing on the result.