Achievement won’t register for legendary campaign

I beat every single level in MCC on legendary. It won’t give me the achievement for it or the halo 5 helioskrill armor for beating them. I’m mostly upset about the amor part because I actively play and participate in arena playlists. Can I get help or tips on how to get all of this to register?

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> I beat every single level in MCC on legendary. It won’t give me the achievement for it or the halo 5 helioskrill armor for beating them. I’m mostly upset about the amor part because I actively play and participate in arena playlists. Can I get help or tips on how to get all of this to register?

I’m having this issue too.
I’ve gone on ‘service record’ in Halowaypoint and it said I haven’t completed The Library in Halo CE which I’ve done both with and without skulls on legendary solo and coop. It’s registering all other achievements except that Legend. I’ve stepped away from it for a few days like other people have said to see if it’s about the registering speed but it simply isn’t working.
Restarting xbox doesn’t help either
Is there something the support team can do about it?

https://m.youtube.com/watch?v=dQwRV-VQsE8

Watched this guy last night with a tutorial on a fix.
Basically you speedrun that particular mission on easy and then quit the game when you finish.
Go to the mission again on the menu and wait. Within a minute all the achievements I earned loaded so hopefully it’ll work for everyone else with that problem.