Eh I at most only have a lot of micro transactions like 10cents each only leh. Then afterwards make payment all failed.
I have money inside Grabpay and definitely daily spend below $5k.
This one is standard card protection, especially is to same merchant. Then after too many tries, Grab will block the card for 24 hours. I believe is when people steal your card, and if they do not know how much limit you have left, they will try to buy things from the same merchant repeatedly.
I've encountered this with Grab card and SCB credit card, for same merchant charging repeatedly in a short amount of time. Then after a few charges, the transaction will be declined.