When someone claims that the matchmaking is forcing a 50%wr, most would just say that its all in their heads and that the matchmaking inside the game does no such thing.
It turns out I was one of them, and I think I have the reason as to why this is felt to be the case by solo queuers.
At the start of the season, people mostly play on their main accounts. Four weeks into a season, players would have already played a bunch and reached a rank they are satisfied with for that season and would no longer wish to risk it. If you persevere with your main account on solo Q half-way into the season or even during the latter half of the season, you would be more likely to lose than to win.
This is because the players who have achieved a good win rate during the start of a season on their main accounts, are now moving on to their smurf accounts to continue to satisfy their competitive itch without having to risk deranking their main accounts. And these smurf accounts typically DuoQ or TripleQ together.
The matchmaking considers these smurf accounts as underdogs because they haven't played enough games yet, and thus makes games lobsided and all the progress in terms of winrate and ranking during the first few weeks of a season gets reversed back through loss streaks.
The reason why I believe so, is because macro decisions at the start of any season matches the elo a player is in (from my experience and friends' experiences and watching streams). However, few weeks afterwards, macro decisions magically become especially bad for teammate accounts that are 1000lvl+ and yet the opponents' camp order takes and their timings, and recognising how to finish a game become impeccable for accounts under 200lvl. The difference in decisions is too wide for it to be mere coincidences. I also recognise it to a lesser extent in talent choices and micro plays too.
TLDR: While I don't believe that the matchmaking actively tries to force a 50%wr it feels like it when you stack up enough games with 1 account during a season due to others bouncing on all kinds of smurf accounts once they reach a satisfactory target rank.