Halo 5: ODST
Believe!
My money is still on a Halo Wars 2 themed Halo 5 update. It's just makes sense, but maybe that's why I'm wrong.
This game's ranking system never makes sense to me, I do so supremely shitty, but Platinum 1. What? That first month this game came out, I got into Gold and worked my way to Platinum was actually nice and steady for me. Every month though, I do my placement matches, and then I proceed to just get outclassed.
The system tries to put you at a 50% W/L ratio. What does your statistic say? Maybe post your GT, so we can take a look at what you play and how well you do.
Also, I'd try the new "Focused" matchmaking preference.