Dear Markus,
we face the same issue: we would prefer to have the field IBAN filled in BUT0BK, too, and not just in TIBAN with IBANONLY deactivated. But I guess we have to cope with this behaviour.
If you activate IBANONLY the field IBAN is filled in BUT0BK. But then the field BANKN will not be filled anymore.
As far as I know TIBAN is filled anyway.
Best regards,
Martin