When having a save prior to sending out the Fleet for an expedition, reloading the save will make the mission unavailable (greyed out).
I had a manual save before I sent out my Fleet on their very first expedition. Something happened soon after, making me want to reload my save. All seemed fine until I spoke to the Navigator to setup the fleet mission again. The top mission was now greyed out, no longer available. I specifically made that save in case anything would go wrong with this very first mission. It is an easy mission as well I assume, considering only 1 star difficulty (top one in green).
I manually fixed this issue by editing my save file, as apparently not all data is being reset when reloading to before activating an expedition. All seems fine again, all 5 missions are available once more. Just too bad it got re-seeded with an even longer duration 4h27m instead of the just over 2h I got before. Fingers crossed my very first expedition goes smoothly