I wrote an explainer at How Competitive Skill Rating Works (Season 13). If you lose more on a loss than you gain on a win, then the game is declaring that you have poor statistical performance with your hero.
However, if you are solo queuing and you are winning more than you lose, the game is wrong. This happens from time to time, is most likely to happen after a rework, and Torbjorn (your current main) did get reworked recently. If you write down your SR after every game it is easier to show a problem (as I did for Mercy a while back: Overwatch Forums). It will probably resolve itself, eventually. Until then, posting in bug reports, especially if you can write down your SR after every game, may help.
P.S. To be precise, what matters is your rank at the end of placements, your record since then, and your current rank. Your max rank is irrelevant and your record including placements has extra games that make things murkier.