YUL-MUC-YUL is a seasonal serivce operated by LH for the last 4 summers and for the period of 1st April to End of OCT.
The plane serving this route is always having first class product as all the flights out of MUC is now having First class.
I flew that flight many times in the last 4 years and load on First is always low 3 to 4 passengers mostly upgrades with miles or cash at the counter or free tickets.
I doubt that LH will cancel the first class on that flight in the next year schedule as low loads show that LH knows from the begining that YUL route is not First class route but they keep sending the plane with First class.
As on arrival in MUC you can use the First class lounge that has schnegen and non schengen sections and it is sinillar in all facilities to the ones you find in FRA. It is very nice and comfy and has everything you need.