Why did my GrabCoins reward tier drop to the base tier after receiving a refund on my GXS Credit Card?

Your reward tier can drop back to the baseline rate (3% in GrabCoins) for two common reasons after a refund or cancellation:  
1) Your spending outside of Grab fell too low: If a store refund causes your total monthly spending outside of Grab to fall below the mandatory S$500 milestone, you will no longer meet the requirements for the higher 5% or 10% tiers.  

2) Your spending inside of Grab fell too low: If you get a refund for a Grab ride, return a GrabMart purchase, or cancel a GrabFood order, those instant GrabCoins are taken back, and that transaction amount is subtracted from your monthly Grab spending total. If this reduction pushes your total monthly Grab spend back below the S$500 milestone (for the 5% tier) or the S$1,000 milestone (for the 10% tier), your reward tier will automatically drop. 

In either case, any subsequent Grab transactions you make during the rest of that billing month will earn the base 3% rate until you hit the milestones again.