Is the fire in Erebus magical? Certainly, the Fire I spell is, but that only creates smoke. Is the subsequent fire still magical? What about the raging, continent-wide forest fire 50 turns later? If every fire is magical, then yes, jungles should burn. If the spell is magic but the fire is mundane, then I don't think jungles should catch fire from adjacent burning tiles (Fire I is still okay, though). Note that I'm not arguing for a gameplay change, just an exposition on the conflicting logic of physics and magic.