How does mmr work? Does it need a fix?

I'm not quite sure how mmr works. I wanted to figure it out so i hopped on my old account from years ago. I was b5 way back then and the account was left as an mmr sinkhole, but to experiment i started playing it again. Well currently the account is 23-3 and bronze 3. How does mmr work. Why can I win 15 games in a row and 8 placements 7 real games, and still end up in low bronze? How does riot expect players in low elo to climb if people who have nearly 90% win rates climb this slow?