So this issue is one that comes from the Rumble Pit Playlist and that can probably be used to an advantage if not fixed quickly.
My friend BA Sangunius and I were going to go into matchmaking in the RP playlist but the only way to do that is if I were to start searching and then invite him and he join while I was searching. It would work as he would just appear in the list of searching players and he’d continue into the game with me. So as we were going to do it again, something happened, I invited him and he tried to accept it but nothing happened when he did, I was still waiting for him then a lobby of 8 players jumped in, so the lobby sits at 9/10. I ask him to join again but instead he sends me an invite to play a custom game and I accept, but as I’m still in that lobby searching with 8 other players the invite does nothing as I accept it.
On my friends other end he is setting up a game to play alone in. It was Zanzibar Anniversary with just regular FFA Slayer, as he’s setting it up I see him pop into the lobby of players still searching. I tell him this but he says he’s still in the custom game lobby. So he starts his game on Zanzibar (Zenith) and as he does that my screen switches to a Zenith loading screen and I just naturally say “Huh, okay, I guess I’m not going to vote for a game” 
He replies with asking me what my screen is on and I tell him that it’s a Zenith loading screen and he’s just like "Dude, I think I just brought the matchmaking lobby into my Custom Game. So the game starts up with the map having no bubble shield activation, and the overshield being oddly placed next to where it’s supposed to be. The gametype settings consist of the regular 3 SMG, Plasma Rifle, and Pistol loadouts that were in custom games and killcams. Sangunius also had the option to kick anyone in the game which he did to almost the whole lobby if they were ahead of him in kills. The game eventually ended and everyone left the lobby. I was freaking out the whole game saying things like “Oh god man we broke the -Yoinking!- game, -Yoink!- it”
So here I am telling all of you and hopefully you Andy if you actually get around to reading this with everything else you’ve been busy with. Anyways, just decided to write it up to notify you of if so you can possibly fix it quickly. Good luck man 
and don’t worry, I won’t be ditching MCC anytime soon. Hope you get everything fixed soon 