Hi Jandoval,
strange - I've simulated a similar situation as you described above in a test system - creating a fixed term depo with installment repayment stream - even you if change nominal amount via changes in capital amount, these installments remain the stable - only final repayment amount is adjusted (as the nominal position after final repayment should be 0).
as the last chance you can also raise OSS note to SAP - in case you don't receive valuable solutions over SDN.
Kind regards,
Renatas