Whenever expenses mileage category is selected in the expense report, the payment method field disappears

Whenever expenses mileage category is selected in the expense report, the payment method field disappear. Is there any specific logic behind this behavior. Prima facie it feels that is a standard bug. If it, does anyone know about the hotfix ?? (The reason for asking this question is, we are trying to configure a expense policy which makes use of the payment method field and because of this behavior our process is hindered.)

When expense category, other than the mileage, is selected, the payment method field is displayed:

When expense category = mileage, is selected, the payment method field disappears:

Any specific reason??