Win rate doesn't matter, what matter is hero proficiencies. All not so hidden stats that make your hero competent for a rank. You could be trying a new hero and the system goes of you general rank. Also winning with new hero give your more rank points, losing penalise you less. Winning with your main give you more promotions but you lose more rank points on a lost.
That is, rank does not depend on winrate, but winrate depends on rank, because if you lose, your rank drops, but if you constantly lose - the game will not try to match you with other players, but will only match those against whom you will continue to lose.
This is called lack of matchmaking - when the game, instead of keeping the winrate approximately equal and at the same time increasing the rank if your winrate increases, throws everyone at you in a row so that you can either completely dominate them, or vice versa, constantly lose.
