"order by" problem with 7.6.00.34

Hello, 
 
I have a strange problem with an "order by"-clause, which doesn't sort as expected. First it appeared in a stored procedure which works correctly with SAPDB 7.4 on Linux. For testing I created a local MaxDB-Installation on my PC (WinXP Pro.) with MaxDB 7.6.00.34 and imported a current backup from my 7.4. database. The "order by" clause looks a little bit "strange", but due to an old design failure I had to go this way.
Here's the Select-Statement:
 
SELECT tbl_Anfrage.Anfragenummer, 
       tblKunden.Kundenname
FROM tblKunden, 
     tbl_Anfrage, 
     tbl_Teile 
WHERE tblKunden.Kunden_ID (+) = tbl_Anfrage.Kunden_ID 
    AND tbl_Anfrage.lfd_Anfragenummer = tbl_Teile.lfd_Anfragenummer (+)
    AND tbl_Anfrage.Anfragenummer Like 'D-%/07'
GROUP BY tbl_Anfrage.Anfragenummer, 
         tblKunden.Kundenname,
         tbl_Anfrage.Anlagedatum 
ORDER BY 
SUBSTR(Anfragenummer,1,2)||substr(Anfragenummer,6,2)||'0'||substr(Anfragenummer,3,2) desc
 
The output looks like this:
D-01/07  XYZ
D-03/07  ABC
D-05/07  EFG
D-02/07  HIJ
D-06/07  MNO
D-04/07  STV

correctly it should be:
D-06/07  MNO
D-05/07  EFG
D-04/07  STV
D-03/07  ABC
D-02/07  HIJ
D-01/07  XYZ
 
If I add SUBSTR(Anfragenummer,1,2)||substr(Anfragenummer,6,2)||'0'||substr(Anfragenummer,3,2) so it is displayed in the result and in the group by clause, it is ordered correctly, but I get some duplicate rows of D-01/07. So I don't know where the problem is and I found nothing in the documenation
 
Kind regards
Peter Hübschen