If you refund a purchase, your points balance might appear negative if the Loyalty points earned from that purchase have already been taken from your account and turned into a Loyalty reward voucher.
Once the points are deducted from your account to create a Loyalty voucher, we can't cancel that voucher. Therefore, you'll still receive the Loyalty reward voucher, but the points equivalent to the refunded amount will be deducted from your account. As you earn more points, the negative balance will decrease.