Interview was made directly with the client - PZU, but Alten HR was present at the meeting. Interview went very well, I've answered all the questions and had a really nice discussion with interviewers - manager and tech-lead from PZU. It seemed like a perfect match considering my skills and their requirements.
After the interview, I've asked Alten HR if my salary expectations are okay with them, and said that I'm open for negotiations.
I've waited for a week or two and received a very surprising call from Alten HR, saying that my interview went terribly bad, that I lack skills and insight, and that the client gave me a bad score and they won't hire me.
I've done a bit of research, and turns out that 50% of the score is due to salary. And because the interview went so well, Alten didn't negotiate the salary with me nor with the client, effectively getting me rejected, despite being the best candidate for the role.