I recently got an ogre on both lava and acid, expecting to do 13 damage. Instead, the acid triggered, doing 3 damage, but the lava did nothing. Is this what the devs intend?
Got changed into only triggering one terrain tile a while back, as to avoid the terrain-cheese that was going on with large mobs. Sad they had to go this way, but bosses was a lot easier than some 1x1 mobs. In a perfect world, varied terrain tiles would work (fire not repeating, but acid triggering alongside et c) - as that'd require some effort on the player's part.
I agree that having multiple lavas trigger would be ridiculous, but having one square of each type would make more sense to me than randomly choosing one.
Sorry, it's not random, as noted in the post by Assussanni. I should have said "arbitrarily" rather than "randomly".