I agree with that SayNoToCarrots, that has been my exact experience with some people I barely know.
When posters are saying it’s an affectionate thing to do etc, my experience has often been with people who I’ve only just met or who aren’t particularly nice to me anyway and who shorten my name to not give me the respect of using my actual name. People shortening my name to be affectionate or friendly are in the minority in my experience.
Having someone introduced as Kathryn to you and you responding “Hi Katie!” is just downright rude and disrespectful. Not friendly. Not a bit of fun. Not your right to decide what someone else is called.
Teachers and staff giving nicknames to children off their own backs is also really unprofessional and I say this as a teacher myself. There’s a power imbalance with little children and authority figures and they often can’t say “I don’t like being called MeggyMoo, please call me Megan.”
And so we begin a lifetime of having to put up with other people enforcing their idea of what our name should be onto us. Those of us that object because we genuinely cannot stand the nickname we are being saddled with are accused of being uptight or rude or frilly because we just want to be called by our fucking name!
If you are one of these people who for whatever reason decide it’s up to you to call people whatever the hell you like, would you consider at least having the manners to say “Ashley, do you mind if I call you Ash?”, then at least the other person has the option of politely declining rather than having to endure a nickname you don’t like.