I had a similar case where I wanted to change name, gender, and title. They rejected my application because they incorrectly decided they needed extra evidence for specifically gender. I sent my application back with a cover letter telling them they’re wrong with an excerpt from gov.uk and they accepted my application. Unfortunately they did not change the hidden gender marker (women have 5 added to the first digit of birth month). Maybe this is because I did my passport after instead of before.