That bit reminds me, why do BATB fans, not to mention Villains Wiki, insist on referring to him as "Gaston LeGume?" This scene, not to mention the triplets' verses in the opening song, if anything strongly implies that "Gaston" is in fact his surname, not his given name.
Yes, Gaston was at one point called Gaston LeGume during development, but if I recall correctly, that was also when he was a marquess who acted as co-antagonist to Belle's aunt Marguerite, and Belle herself had a younger sister named Clarice, and Maurice was a merchant like the original tale, NOT an inventor, none of which were retained in this film.
I know that addressing someone with Mister or Miss followed by their first name used to be a thing in the Netherlands and Belgium, when addressing or referring to colleagues for example. I'm sure they also used to do this in France. I know there's a French comic called "Monsieur Jean", which seems to be his first name.
I see it's also a custom in the Southern United States.