Is there a grace period for data bundle that got expired?

As long as you renewed your data plan within 7 days after expiry, you're all good. However, for you to get the previously unused data balance rolled over, you must renew a similar plan. That means if your previous plan was a monthly plan, the renewal must also be a monthly plan. You will not get the unused balance rolled over if you renewed with say a weekly, 2-month, 3-month, 6-month or 1-year plan. It's got to be like for like.