Wut? According to who? Is that a legal stance or your personal stance? Seems bonkers to me.
My Dad was born in the UK to a Polish father and a British/Indian mother, but he's always had a British passport, and there's no question of his nationality.
Why are you German? This may sound daft, but you are not your mother![]()
Not filling the right forms at the right time I'd guess, we don't automatically become citizens for being born on UK soil, unlike the USA for example (boris johnson was, until recently, a US citizen because he was born while his parents were over there).