AWT at payment time for a past range

Hi,

Please tell me why the AWT set up at payment time and Withholding date basis is set as "Invoice Date" in the Payables Option for the following scenario is taking into consideration the payment_date and not the invoice_date setup in the Payables Options:

Rate Effective dates from to
-----------------------------
19.5    01.01.51    31.12.2015
19       01.01.16

Invoice is created in 2016 with Invoice date smaller than 31.12.2015.

If payment_date is also smaller than 31.12.2015 the AWT is taking the 19.5 rate, but when the payment is made automatically the rate is 19 and it takes into consideration the payment_date, even if the Withholding date basis is set as "Invoice Date" in the Payables Option.