When you buy something with UNi Card, the merchant requests payment from the card issuer, not from you directly. The card issuer pays the merchant and adds the purchase amount to your account balance. You then pay the card issuer.
Returning a purchase works the same way but in reverse.
When a retailer issues a refund, the money doesn't go directly to you. Instead, they ask your card issuer to credit your account for the returned amount. The card issuer then posts the credit to your account.
In case a refund was processed by the merchant and not credited to your UNi Card, please submit a request to the Customer Support Team by navigating to Support → Chat with Us in the App.