Order Total In Secondary Currency

If the there are two or more currencies in the shop the Total is always displayed in selected currency and the amount is calculated based on current rate.

If, for example, the rate of secondary currency was doubled since the order was placed and paid, the total will change respectively regardless of order status.

In my opinion since the order is paid its total in all currencies must be archived (or, let's say the total in primary currency and all exchange rates, or maybe only the total in primary and payment currency)

Is that a bug or "the way it operates"?