I feel you.

Prepared 1 year for my dream job and got rejected due to a bad technical question given to me. I was told not to use regex as it overcomplicates the solution, so I struggled the entire time trying to think what could be simpler than regex. In the end the "model answer" was to parse character by character, which spoiler, isn't simpler than regex.

The interviewer was also quite junior so I was judged entirely on that one question. None of my other skills mattered. It's been a month and I'm still upset and salty over it.

I don't have any advice for you, but you're not alone in this. Sometimes it's just pure bad bad luck of getting a poor interviewer.