Technical issues with Heroes III.

I once encountered this issue. I completed the "Stubborn Hero" campaign in Heroes of Might and Magic VI (the latest version), and when I tried to save the game after completing 6 campaigns, a 7th campaign, "Seeds of Discord" from the Restoration of Erathia expansion, appeared. After completing it and saving, only the 7 RoE campaigns were saved, and the Heroes of Might and Magic VI save disappeared. Both Heroes of Might and Magic VI and the Restoration of Erathia expansion are included in the game build. What caused this, and what can I do to prevent it from happening again?