INZ normally only require a further medical certificate sooner than the 36 months if they have some reason to think that the applicant's health has changed, or has been at further risk (e.g. by visiting another country where serious disease is endemic) since the earlier medical. Perhaps the people you have heard about have been in one of those situations.
Or perhaps you're getting the sort of dig that many visa applicants do. It seems it is human nature for some people to tell anyone doing something out of the ordinary, or enviable (like being in a position to go for residence), about all the worst case scenarios that can possibly arise, often adding, "That's what happened to my workmate/my cousin/my neighbour." It's as if they don't like to see somebody with a shiny hope they haven't got themselves, so they give them a twist of extra anxiety. This behaviour has been seen from people's family and friends, even, and is very widespread on some online forums.