Air Asia — Premium Flex date change

Booked a premium flex ticket (Pnr- S92G9X)from Bhubaneswar to Bangalore on air Asia flight. Then rescheduled to a new date and within 5 minutes realised that the date was wrong. So tried to reschedule again as I knew in premium flex rescheduled can be done again without any charge only difference in price should be paid. While second rescheduling to my surprise the amount I had to pay is ₹3796 instead of ₹777, the difference in price amount. I tried talking to both air Asia and easemytrip customer care. Air Asia person told me that while first rescheduling my premium flex ticket has been changed to economy fare. How that has happened ? Because while rescheduling there was no option asking whether I should book in Economy or Premium flex fare. How the hell I would know that automated without giving options it will be changed from Premium flex to Economy fare.This is a flaw on your rescheduling page. Why should I pay for your flaw?