Mont doesn't like surprises. he just can't deal with them and gets very aggressive when things are sprung up on him even if it is a nice thing. so Ferre takes it into account with pretty much everything, always making sure Montknows what he is walking into. And three months before Ferre proposes he warns Mont gently like "hey just a heads up I am probably going to ask you to marry me soon" and mont is like "hey just a heads up in probably going to say yes". And they both just go about their day
Oh my goodness thats adorable. Like Montparnasse knows in the back of his head that there’s a proposal probably on the horizon, but when it happens it’s still like ‘oh my god, he actually did it, he actually proposed’, and he’s super happy and says yes and then has to smooch Combeferre lots because of how happy he is.