Store Credit for WooCommerce

If the order is refunded that was paid with store credit, then the amount can just be lost. It is only restored if the coupon still exists, or if it is still in the trash.

If a partial refund is done then it is totally skipped, there is no support for this.

If the refund is done and the credit has expired then it is just restored to an expired voucher. There is no mechanism to allow extending it by an amount automatically.