Last December, I was on Lufty to MUC and was on a remotes stand for the out and return. Considering they had the whole of their new terminal to play with, I presumed it was a maintenance or construction issue.
The key point for pax and bussing is to be LAST on to the bus. get jammed in by the door and even if the doors open on the other side, you'll be off faster. On some regular journeys in the past, I knew which side the doors would open and could be first off with a run!
Yes last December there was construction work going on as I believe is still now to expand the LH terminal. Not sure why they used a bus at LHR never happened to me with them there. I have been bussed by LH at MUC for the Bremen and Toulouse flights not very nice in winter but that's life I guess.