As you can see in the image below the mission is there but not visible (missing icon). It may be because there are more missions in the same location.
I was able to reproduce it by selecting the mission and then going back to map from briefing screen - then the icon disappeared. This may be the issue for missing missions for other users that are not using Mission List.
The icon did not appear after going to HQ and back to map.
Correct way should be to show that there are more missions available at the same location as it works for other types of missions.