Assault on the Control Room Completion problem

Assault on the Control Room will not register as being complete for me. I have played it on Legendary co-op and single player. The co-op completion will not register at all, and single player shows compete on Normal only.
Any ideas? I have completed every other mission on Legendary. Anyone else having this problem?

it shows single player completion but not co-op completion?

Its possible you lagged out of each other’s game if you were playing over XBL.

My friend and I had this happen playing the Library. On his screen, I died. He kept asking me if I’d respawned while I was killing flood. Apparently we played for several minutes in two different games. In my game, he was running around shooting, and in his I was dead on the ground.

So I could see a scenario where that happens as you finish a level…and to you it would appear as if you’d finished it on co-op but the network says differently.