Question on ranking

So I hit Onyx with swat today, and I’ve won every single game since becoming Onyx. Then why the hell have I decreased in rank?

My bet is that you have increased in rating and that you’ve misunderstood how Onyx works. You get to Onyx and you’ll get rating 1500. Highest rating is 3000. The 200 players with the highest rating at any given time belongs to the Champion tier.