"HW rulez" mod (add-on) for HoMM3.

After all, this is essentially the same oversight as with the Nagas, when they were given sulfur instead of dragon stones.

The lava isn't a mistake; on the contrary, the developers clearly did it intentionally so that the Dungeon battlefield is always used in battles within the Dungeon (if you look at the code, it's incredibly unlikely that they made such a specific error by adding a meaningful extra check, unlike the sulfur in the Nagas, where the cells in the table were simply mixed up). They were probably not very concerned about the native terrains but were more concerned with ensuring that the lava (as well as all other) battlefield with a sky is not used in battles within the Dungeon. The situation is more like a bug with the courage icon, but with the lava, there is an explanation for why it was done, while with the icon, there isn't.