When registering a benefit for card payments, with limited installments quantity (for example, "10% cash"), the discount is presented after selecting that amount of installments at the checkout. However, this discounted value becomes the new total value, on which the other installments will be based to present values. Then, in the selection field the other installments which are calculated based on the total value with the discount are displayed among the options. When you select a different installment, however, the displayed value becomes the correct one.
This scenario only happens for card discount and at the time of selection. The purchase is never closed with the wrong value.
There is no workaround for this scenario.