Not having all the information, it’s interesting to ask ‘who made the mistake?’

Did MUC get confused with their own rules? I think that’s unlikely. We can hope they know their own rules and they seem to deal with them regularly. The OP tells of another time he was flying to MUC only to be diverted.

Did BA misunderstood MUC’s rules? To me this seems more likely. I’d guess there is no staff member well informed on MUC - airlines don’t have these specialists. Flight and cabin crew took no heed of passenger’s warning.

Whay probably happened was MUC said you can fly in late. BA assumed this meant any time. MUC assumed BA was aware that anytime means anytime before 12.