Halo CE - Splitscreen progress didn't save

I completed the CE campaign
(Xbox 360) in Splitscreen. It displays completed on Coop Legendary for every mission for my friend (guest profile), he also got the legendary achievement upon beating the last level. I didn’t get it, looked at the menu, now it only displays the missions completed on coop legendary we did today. (The last three) The others ones display nothing in the coop section, as if we never played it. Extremely frustrating, because I don’t want to do most missions all over again. Any ideas?