I’ve been playing Halo MCC to get Helioskrill armour. Went through the entire Halo 1 Campaign with friends and solo (mixing between). Well apparently 6 of the missions didn’t record my stats, and I didn’t receive achievements for completing the missions on Halo Waypoint and in my Career stats in-game. But when I go to the “Select Mission” area, the missions show that they’re completed on Legendary (some completed both in solo and co-op). I just completed Truth and Reconciliation for the 3rd time and it’s stats STILL haven’t been recorded or gotten the achievement for it.
Try playing the missions without any skulls active.
However even if you do get the legend achievement there is a possibility you won’t get helioskrill due to numerous others experiencing technical difficulties to get it to unlock. Don’t worry though it’s in the req pool.