I've advised earlier in this thread, GrabPay doesn't automatically process the unposted or refunds charges. That's why, it's very risky to use GrabPay MC, for transactions that may be refunded or voided.
GrabPay puts the responsibility on its customers, to report to their Support, on transactions that are not refunded, and they required customers to provide proof of acknowledgement from merchant that the charged amount will not be posted and refunded. Without this merchant confirmation, GrabPay will not auto process the refund, most of the occasions.
GrabPay is still not ready, to run their role as a Bank or Credit Card provider. Their Support and internal processes are not familiar with many bank and credit cards typical issues. In fact, some of the US and SG websites, still doesn't accept GrabPay MC..