Looking at the postal code boundaries on maps.google.com it seems that the postal code overlaps several localities:
https://www.google.es/maps/place/Telangana+506366,+India/@18.3178586,79.5682081,12.25z/data=!4m5!3m4!1s0x3a3324c84ccde691:0xdedd70d16ab9cfab!8m2!3d18.3214924!4d79.616521
In such situation Geocoding API doesn't return a locality in address component because of ambiguity.
However, I would expect the postcode_localities array in the response that according to the documentation
is an array denoting all the localities contained in a postal code. This is only present when the result is a postal code that contains multiple localities.
https://developers.google.com/maps/documentation/geocoding/intro#Results
At this point it looks like a bug. I can suggest raising an issue in the public issue tracker: https://code.google.com/p/gmaps-api-issues/