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As far as I remember, you enter a cheat code to get 0 points for the map, but you can still earn points in other maps of that campaign. I don't know about Heroes of Might and Magic III. I had a similar situation in a Barbarian campaign, I think it was a vanilla one, where you had to kill all the enemies. I cleared the entire map, turned off the fog of war, and there was no one left, but the game didn't understand that there was no one left. I got frustrated and entered the Valhalla cheat code.
In general, after my experience with the Bughesda games, cheat codes are a real tool for me to complete the game. Sometimes, you can't do without them, because the problem might not be with your skills, but with the developers >_> And Heroes of Might and Magic IV is also a bit buggy, although not to the level of the Elder Scrolls or Fallout. Again, comparing it to Bughesda, there are bugs on top of bugs in unofficial add-ons and mods, and bugs chase bugs, I think the same is true for Heroes of Might and Magic IV (although I don't complain about Equilibris, and besides it, I may have only played a couple of amateur maps a long time ago).