Inferno on the surface.

It's hard to say whether it's a bug that any landscape underground is a dungeon landscape.

This isn't a bug – there's a specific check for the underground level, and if the battle takes place there, the ground is replaced with a dungeon landscape; it's unrealistic to think that this could be added to the code "accidentally." The logical reasoning is also quite clear: the developers were primarily concerned with ensuring that the battlefield image in the dungeon showed a dungeon, and not soil with a sky in the background; what kind of native ground it is is a secondary concern.

In a sense, this can be considered an oversight – instead of creating new battlefields for territories in the dungeon, NWC didn't bother and simply replaced the ground type. But to say that this is a bug, i.e., it happened by accident and was not actually intended – would be incorrect.
In Heroes, by the way, a new battlefield for underground lava has been created. :)