The system is not sending the partial cancellation of a Giftcard transaction. We also found a scenario where the same thing happens, however the cancelation works. This leaves the refund amount higher than the transaction amount because the value refunded is the first partial refund + the second refund with the full amount.

Find a transaction where a partial refund/cancellation was requested using Giftcard.

There's no workaround, however, the user can add the refundable credit via admin UI.