The interest income should be tax related and the transfers to reduce the principal should not. There are a number of ways the transfer may have been set inadvertently. If the transfer is set as tax related in the payment transaction, reminder or memorized payee, open the split, select the transfer, Edit > Tax line item assignments, the blank entry in the Tax item: pull-down menu, and OK. If the transfer is set as tax related in the loan payment details, right-click on the transaction and select Tax line item assignments, the blank entry in the Tax item: pull-down menu, OK, and press Ctrl + Enter.