I switched between co-op and solo (My friend had to go then he came back) for missions 3 and 4, didn’t receive the achievement for either of them. In game I had all the rally points available to go back to however according to it I didn’t complete them on any difficulty. I just checked my halo 4 stats on halo way point
and NOW it says I completed the missions, but still no achievements.
lazy people TLDR: did campaign missions, no achievments, y u no