If the inbound LH flight diverts and your flight is cancelled /significantly delayed, I would ask the airport staff at MAD about your rebooking options. Anyway, the rebookings will be made a some stage by LH back office if you are un-successful at MAD, so keep an eye on your booking on the LH app or website. If no LH staff at the airport (outsourced staff only) it may turn out difficult to get them to touch your ticket - in particular if the flight is only delayed as they will refer you to deal with it at MUC. I don't think it makes any difference that you are in J.