I recently completed all campaigns of the Halo Master Chief Collection on the legendary difficulty. When playing the games through, I did not receive the achievements for Forerunner, Infinity, Composer, and Midnight which halted the unlocking of all associated achievements for completing the games on legendary. I jsut contacted Xbox support and they said it will update and I should now receive the Helioskrill helmet i deserve in Halo 5 and hopefully unlock all the achievements as well.