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

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