How to return Char Array Value in Arduino IDE Function?
char char_array[str_len]; is local to the function - it is created on function entry and its memory is released on function exit. The contents of that memory are undefined once it has been released and its data may or may not be valid.

If the caller creates data and passes it to the function, it survives the entire function call-and-return, and the caller will find the correct data in it. Another (less elegant) way to do it is to make data a global object, which again, gives it a lifetime greater that that of the function, but globals are subject to accidental over-writes and abuse by other functions, so they are not recommended unless they are truly the best solution in a given situation.