Royal Enfield — chain set gone within 1 year

Address:

Chandigarh, Punjab, 160071

It's just been 1 year when I bought my classic 350 from Chandigarh sec 32 .
The teeth of rear set broke and chain is making a lot of noice.
All the services are done from agency in Industrial area and they say it's mileage is perfect
Logically if mileage is perfect means it's not driven harshly and speedily so how can spoke be broken within 12000kms.when I asked them they say ur lucky it's hasn't happened at 8000km or so
Is this the explanation they could give
And from then chain is not working and making a lot of noice and I am afraid I can lead to miss alignment of the bike.