I recently completed by meta full loop for E4/E5 for the London office. For the final loop:
R1 Coding: I was asked 2 questions and I was able to code both of them with all edge cases/followups. This interview went very strong.
R2 Coding: I was asked 2 questions and I was able to code both of them with all edge cases/followups. This interview also went very strong.

Behavior: I had a great conversation with the interviewer and this round went very well

System Design: The system design round went well and I was able to do 2-3 deep dives. The question was around Designing a post system, very open ended.

The recruiter came back a week later and to my surprise they wanted a follow up coding interview. I had completed by coding rounds about a month back and wasn't looking forward to re-visiting it again. I tried to guage why the follow up coding but didn't get anything concrete except that one interviewer was not convinced.

Has anyone had a similar experience? Is it not strange that perhaps a personal bias of an interview can result in this situation even when the interview went very well? Any suggestions on what I can do about it (along with giving the follow up interview)?

Thanks!