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

Then, explain at the same time why the overground map editor shows underground lava.

Because the map editor determines whether to open a cell or not based on its terrain – that's how it's designed. However, the mechanism for determining the terrain type in battle is a bit more complex: during a siege, the city's native terrain is set; in a battle on the coast, sandy terrain is set; and in the underground and in the mine (crystal, gold, and abandoned) – underground terrain is set.

Regarding whether the lava situation in the underground in battle is correct or not, and whether it should be changed in SoD within the framework of HW_Rulez or otherwise – I don't judge.
I'm just commenting on the opinion that this is a bug, based on my knowledge of how it actually works internally.