In both cases (MUC-BRS and FRA-JRK) the metal was BMI Regional (BM) and the operating airline was BM. It was only an LH codeshare.
What seems to be a difference here is that I have LH*G (SEN) status, and fassy has another airlines *G status. So maybe it's only LH SEN have access to LH Business Lounges at FRA / MUC when travelling on an LH codeshared flight operated by BM.
Who knows. I can't find any written rule about it.