If you have received a refund your rewards balance will be reduced.
For example, a recent $100 Tap & Zip transaction received a $2 cashback (2% of the order value). If a full refund is issued the $2 cashback will be deducted from your rewards balance.
If a partial refund of $50 is issued, the cashback value would then be reduced. 2% of the remaining order value would be $1, meaning the difference of $1 would be deducted from the rewards balance.