@2J&D
The ticket was issued on LH (220) stock, using one of their published fares for BRS-BNE (-RCGB type fares). The routing rules include:
BRS-LH/BM-STR/NUE/MUC/HAM/HAJ/FRA/DUS-SN/OS/LH-ZRH/VIE/
MUC/FRA/DUS/BRU-SN/OS/LH-BKK-TG/LH-BNE
The problem was that LH re-issued the ticket (after TG cancelled the BKK-BNE sector, and I had to re-route BKK-SIN-BNE and onto SQ), and the re-issued ticket didn't make it to BM correctly.
BM have confirmed through check-in and interlining of the bags will be fine. So all good news in the end.