Checkout Currency changed from USD to other currency, but rate not applied

Hello, I have actually disabled checkout in other currency. Although 95% of all orders are currently being paid in USD (default currency), there are some 5% of orders randomly being paid with other currency, and furthermore, the rate is not applied causing us huge losses. Attached is screenshot showing an order for a product fixed at USD29.95, which is charge at ZAR 29.95 instead. Please advice.