Most accurate way is to check the "View activities" on the UOB app once the fraudulent transactions have been refunded (credited).
The fraudulent transactions were posted in the month of March so it will be taken as extra spending in March. However when the fraudulent transactions gets refunded later (say in April), it will be taken as a credit which will offset the total spend amount for that month.
Example if you spend $1000 in March (all transactions posted successfully) but then you refunded an order that is worth $500 and it gets credited only in April, UOB will take it that you still spent $1000 in March and -$500 in April.
Your point is also valid. It depends how UOB handles it.
UOB can keep the monthly spending in April as $0 and clawback the fraudulent transactions cashback in April. This will only affect the value amount of UOB One card and not the monthly spending in April.
Example based on the assumption that the fraudulent transaction amount is S$200
31 March
Monthly Spending: -$900 (including fraudulent transaction amount of $200)
Cashback: $29.97 (based on 3.33%)
01 April
Monthly Spending: $0
Cashback: -$6.66
UOB One Card: -$900 + $700 (assuming paid for legitimate transactions) + $29.97 (cashback) + $200 (assuming UOB offset the fraudulent transactions) -$6.66 (clawback on cashback)