When you enable the feature "(Italy) Separate accounts for credit note" and you post an invoice from a return order the system is not using the account for the credit note but always the one for the revenues because at the creation of the return order the system populates the field “Main Account” in the Line details > Product with the main account used in the sales order (revenues) and uses that account at the posting.
In order to use the account for the credit note you should delete manually the value in the field “Main Account” from each line of the return order.
This is not an acceptable solution in case of orders with a lot of lines.
At the moment of the creation of the return order, if the mentioned feature is enabled, the system should not fill in the account of the sales order in the "Main Account" field.