My mother chose names for us that she liked, but that she thought could not be shortened in any way, so was infuriated when she discovered that people (twee people, as she always referred to them in her rage) actually lengthened them, so that Jane became Janie, for example. When one of my sisters started school at 5, and I was 6, I was hauled into her classroom to deal with her on behalf of her teacher. She had called my sister a lengthened version, and she refused to answer. For the rest of the day she refused to speak to her at all. The poor woman's name was Mrs. Pegg, and for the rest of the year my stubborn little vengeful sister called her Mrs. Peggy.
My son is Robert. Older relatives on DBH's side have tried to call him Bert or Bertie, he is always Robert to us and I would have been as livid as the OP if they had kept it up. My SF tried Bob a couple of times and was glared at by DS, but now he is older his friends all call him Rob, and he is happy with that. I found that if someone doesn't like what they are being called, they will soon sort it out for themselves.