The tying of "Sieur" to the first name is a reflection of British titles, I think. I don't know how it worked in France.
I do know that Alan Sugar (the guy who does The Apprentice - You're Fired in the UK) was Sir Alan after he was knighted; then Lord Sugar when he was made a peer; then Baron Sugar when he was made a Baron.
Obviously he played his cards right with the Council of Brilliance.
Smells like favoritism to me.
Dementlieu suffers from being given titles initially that reflected a British Peerage, but then later tried to adopt the French style of Nobility. I'll put this down to the simplest level I can, I think; Maybe.
In France, you weren't called by your name or even your Household name. This is also the same for England; Louis de Bourbon was never called Monsieur de Bourbon in his life, nor was Philippe de Bourbon, the Duc d'Orleans, his brother. Louis, as King, was Louis XIV, Monsieur le Roi. Phillippe de Bourbon would be called Philippe by his friends, but Monsieur le Duc and the Duc d'Orleans by anyone who would refer to him formally. Chevaliers would often be referred to as Chevalier de X, Chevalier, or simply, Sieur X. It depends on the setting they're in. The trick is, a Chevalier in informal dialogue is never just, Jean. He's always Sieur Jean, or the Chevalier de X.
The problem PoTM has, is that they've instituted the strange tradition where everyone is just a Chevalier de la République, rather than having something important attached to the name, which cheapens the value of it and means that they can only be called Sieur Jean in distinction from, Sieur Jean Bernard, Chevalier de la République. Traditonally, one would be assigned a title of Knighthood with either the location of their great deed, or more rarely, a tract of land afforded alongside it. Thus is why Alix Sinclair Martineau, when she initially received her titles, was, not Dame Alix, but the Dame d'Estellier; For her tract of land, Estellier, was given with her title. Likewise, if a Chevalier was given a territorial designation, it would be far more recognizeable to them to be known as say, the Chevalier d'Avejan, as Sieur Jacques Boucher was once.
Land is more important in many cases than a title, and the more you can brag about owning land and make other people acknowledge it, the better off you are.
In the English system of Nobility, their Noble assignments are all surrounded to positions of the Royal Family. Thus, the Prince of Wales, the Duke of Norfolk, and so forth have historically been revoked and issued at the King's discretion as befitted his arrangements. However, in the English system of Nobility, they never referred to them in regards to their land, but rather, His Grace, His Grace the Duke of X, etc. They never used a name, because they are being impersonal.
A man who has land, and a title, is more important than the name of his family, and his first name would be a gross overstep. You're supposed to be aloof, disconnected, and pointedly not personal with anyone not in your immediate family. So if you try to just make everyone a title then their surname, you're ignoring the fact that they may or may not own land, and may or may not be better than you.
This is already the case of landless Baronetcies, which are technically an ENGLISH title, I'm looking at you, Dev Team. But if you're a Baronet without land, you're just, Monsieur le Baronet [Surname]
So Monsieur le Baronet de Bernard would be suitable. Or Baronet Johnson, if they don't use a 'De' in their family's name. All other titles beyond Baronet are required to be associated with a tract of land, and that tract of land is infinitely more important than their family names in terms of prestige.