The headhunter says "mixed reference," and you have translated that to "negative feedback," which is not the same thing. It could be that someone who generally

The headhunter says "mixed reference," and you have translated that to "negative feedback," which is not the same thing. It could be that someone who generally thinks highly of you gave a glowing reference, but once the headhunter or hiring manager started digging into specifics, the person softened, hesitated, or otherwise answered in a way that the company was not looking for. It's possible the reference was unaware that what he/she said was interpreted in a mixed or negative light by the company.