Do most credit cards consider grab top-ups as a transaction?

I suppose you meant not a transaction that can be eligible for miles/rewards?

It depends on the MCC code for the merchants.

The popular UOB one to grab TOP up will not be eligible for quarterly bonus spending minimum with effect from 16 March.

Yes, grab top-up is a valid transaction. However, whether it is a valid transaction for getting the card's reward will depend as different credit cards have different terms and conditions. The best way to check this is to read the respective terms and conditions of the card itself, specifically under its exclusion clause.

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