I don't think it is LH's Senator Service's fault for refusing to change your routing just because you bought a ticket to originate from MUC when the ticket clearly indicates that you should be terminating at BCN. In fact, you intended to skip the last portion of the ticket and no-show the flight.

Basically why didn't you book you ticket as an open jaw in the first place? Perhaps you had exceeded your stopover options? It is not LH's responsibility to see you get on your flight at MUC, but rather to get you to your FINAL destination which is BCN. Rerouting your flight via any destination as long as you get to BCN is their priority which is what they have to do.

If your final destination as booked had been MUC, then LH is obligated to get you to MUC no matter how they route you! But it's BCN.

SO you have 2 options. 1) Get off at FRA and buy a seperate ticket to MUC, or 2)Reroute your purchased ticket to start in FRA and get off in FRA on your award ticket.