This is my personal view about this issue.
This 658 cc engine is not enough to battle with the mountain roads and to win the hilly road, the alternator assist to add more power to the engine . when add more power to crank shaft pulley from the alternator, the belt and tensioners runs with load and this may be the reason for the tensioners less life time.
Also these parts may not pure japanese(not made in Japan) as many factories in China, India, Korea, Thailand , phillipines made many spare parts for most models in Japan as well as European cars and these parts may bring the troubles .
We cannot say these factories follow all the standards in production or may be production faults. . Now many vehicles import to Srilanka(Specially Japan domestic models) bring many issues . In Japan they use maximum 2 years and during their usage they do not any as mother company has some what warranty for their cars, But after complete 2 years , The companies do auction for these used vehicles and they resell to countries like SriLanka .
Now most people loses their confidence about Japanese vehicles too ,The car sales owners too only sell the vehicle and after that forget the customer or many fairy tales or some. At the end the customer helpless and cannot drive with confidence.