Similar thing happened to me on my 1799 prepaid plan that is not eligible for unlimited free 5g trial. However, the Airtel app showed it is eligible for my number. Trusting Airtel, I ran a speed test. 1GB was over in less than 30 seconds. After a few hours, I checked the remaining data. 1GB was wiped out out of 24GB for the entire year!
Mine was prepaid. I can understand how frustrating it'd be for a postpaid user who had to pay a fortune for Airtel's mistake.