View based on view with concatinated fileld returns a null value

Why does the below view based on vw_1 bring back a null value (Note: Vw_1 does not return null value on Prop_Name on any row)  - SQL Server 2008

----------- Prop Name Returns Null Value --------------
SELECT     Prop_Name, PropID  FROM         dbo.vw_1
GROUP BY Prop_Name, PropID

----------- Prop Name >>Does Not<< Returns Null Value --------------
SELECT     Prop_Name  FROM         dbo.vw_1
GROUP BY Prop_Name

----------- Prop_Name  --------------
dbo.tbl_Properties.PropName + N': ' + dbo.tbl_Properties.PropAddress + N': zip-' + LEFT(ISNULL(dbo.tbl_Properties.PropZip, N'') + '000', 3) AS Prop_Name

----------- Entire View #1  --------------
SELECT     TOP (100) PERCENT dbo.tbl_Properties.PropName + N': ' + dbo.tbl_Properties.PropAddress + N': zip-' + LEFT(ISNULL(dbo.tbl_Properties.PropZip, N'')
                      + '000', 3) AS Prop_Name, dbo.tbl_Properties.PropID, 1 AS Root, LEFT(ISNULL(dbo.tbl_Properties.PropZip, N'') + '000', 3) AS PropZip_3Digit,
                      dbo.tbl_Prop_Agreements.PropAgr_StatusIdd, dbo.tbl_Properties.Prop_StatusIdd
FROM         dbo.tbl_Prop_Agreements LEFT OUTER JOIN
                      dbo.tbl_Prop_Surfaces ON dbo.tbl_Prop_Agreements.PropSurfaceIdd = dbo.tbl_Prop_Surfaces.PropSurfaceID LEFT OUTER JOIN
                      dbo.tbl_Properties ON dbo.tbl_Prop_Surfaces.PropIdd = dbo.tbl_Properties.PropID
WHERE     (dbo.tbl_Properties.PropID > 0)

Thanks in advance for any help.
LJG