If you have an unfilled close-to-expiry option order on the day of contract expiration, Futu will calculate the margin required for the exercise and settlement of the option assuming the order is filled. If the account becomes risky, Futu reserves the right to cancel such orders.
Usually, when a close-to-expiry option order is cancelled, it may be because you submitted the order before the expiration risk control was running, but that pending order has subsequently triggered the relevant alarms as time moves closer to market close.