Weird SQL Behaviour - or is it just me?

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SQL

I have a stored procedure (Sql server 2008)

BEGIN TRY
  DECLARE @result Int
  DECLARE @test Int
  Declare @now datetime
  set @now = GETDATE()

   Select @test = COUNT(*)
   From dbo.Table1 as M
   Inner join dbo.Table2 as L on M.SomeKey = L.SomeKey
   where M.Id = @Id
   And IsNull(L.DateTimeExpires , @now) >= @Now

 if @test = 0 -- no valid record found
   set @result = 1
else
    set @result = 0

END TRY	
BEGIN CATCH
  set @result = 2
END CATCH
	
select @Result

I am calling it from a bit of VB.Net as follows...

Dim db As Database = DatabaseFactory.CreateDatabase("MyDatabase")
      Dim dbCommand As DbCommand = db.GetStoredProcCommand("MyStoredProc")
      db.AddInParameter(dbCommand, "Id", DbType.String, computerId)
      Dim OK As Integer = db.ExecuteScalar(dbCommand)
      Return OK

Which is (and I don't think this matters a jot) on the server side of a WCF service.

If I run this, it returns '1' - i.e. it finds no records.
If I run it in debug, wait for, say, 5 seconds before calling the ExecuteScalar, it returns '0'

This is consistent (rebooting etc. makes no difference.

So - I narrowed it down to the fact that it must be something to do with the SQL
And IsNull(L.DateTimeExpires , @now) >= @Now

If I replace this line with

And (L.DateTimeExpires is Null or L.dateTimeExpires > @Now)

then it seems to work fine regardless of any delays.

The DateTimeExpires column in the appropriate record is null.

I am sure I am just missing something obvious - but I'm blowed if i can see it...

What's going on?