IMO it doesn’t really matter what you said the method was for. If you change the format of a string that is returned by a method that returns a string, there’s a risk of breaking user code, even if it’s just in the context of their dev environment.

Philosophically, whether or not the behavior of your API has changed is completely disconnected from whether or not others are using it “right”. If I can depend on a function to return a certain type of value when given certain arguments, and if it doesn’t produce other side effects, then it doesn’t matter what the docs say or what the function is named, I can use it in any context where I need that type of return value and have this type of arguments available. This type of function is just mapping data to other data. If you modify the function in such a way that the return value changes after being given the same arguments, that’s a breaking change in my book.