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Is it correct Subject for this letter ("Misbehaviour Branch Manager")
No, this is quite patronizing, and should only be used of children, if at all. Also, it is not your place to judge or suggest chastising anyone except your own subordinates; merely state the incident and let the appropriate authority deal with the situation.
Your letter does not need a Subject line; most letters do not. If you feel that one is necessary, then something like:
Difficulty with Online Banking Application.