Hard reset your Xbox. I know it sounds simple, but hard resetting my Xbox after every Halo: MCC update has solved numerous issues for me. And I don’t mean simply powering off your Xbox, I mean holding down the power button for 10 seconds before starting the console up again shortly after. Before the hard resetting, I wasn’t able to join anyone, the game was constantly crashing and I encountered countless bugs.
Believe me, it fixes a lot of things. Try it and post your results here.