While playing offline, I have experienced across all Void Campaigns I have played to date (Ivory, Ebony, and Viridian) the same issue. Whenever a mission is supposed to open due to completion of a specific mission (no other special criteria),  said mission does not open. For Example, in the Ebony Crusade, the Secret Mission at the bottom center of the map is supposed to open when the mission to the left and slightly above it and the mission at the top almost directly over the previous are both complete. So far, in 3 play throughs of Ebony, on both hardcore and normal, this has not happened.