What evidence are you proposing that, this is not happening time to time in BL? It has been observed that if you are on a win-streak you start getting very unfavourable(according to player's current rating) games and rise really fast if you win these unfavourable seeds.
On a side note, Competitive Overwatch Streamers has taken accounts that were "stuck" in bronze/silver over a year and rose them up to Grandmaster before.
Finally, I have no idea if you are completely ignorant of "Game Theory" or gone mad because you studied it too much, but I would like you to define this theoritical (with mathematics not ideas) match-making system that does not include a MMR.
What is it? a State-Machine? a Neural Network? a bird? a flying man?
I don't know exactly what is happening in Bannerlord. But I know the design trends of online games, and I see Bannerlord falling into a really terrible one that I am familiar with. It sounds like you are also familiar with 'performance-based' ranking systems. Do you know this from Overwatch?
Part of my thesis is that we cannot count on PBSR and other systems to make accurate judgements about player skill. I believe that should be measured by the match result and the win/loss record of each player. The problem with MMR and other algorithmic handicapping systems is that they pervert the match result, making it intrinsically unrelated to the measurable skill differences between each player, and necessitating performance-based rating in the first place.
By the way, these systems are not comparable to ELO because ELO is designed for tournaments of a 1 v 1 game. MMR, EOM and other algorithmic handicapping systems are being implemented in games that have many players on each team, and are not true tournaments. The multiplayer aspect is crucial, because algorithmic handicapping systems depend on multiple players to balance the scales.
The alternative I propose is simple and obvious, to anyone with the slightest grasp of game theory. Matchmaking for ranked competition should be based on rank, and it should be solo-queue only. Rank should be based completely on match results. I.e., did your team win or did they lose, what is your win loss record? Anything else is a travesty.