It's good to jump straight to insulting, thanks buddy.
In any case: It's not a bug, it's a feature.
Oh, that wasn't an insult. Its simply common sense so assume it'd be understandable for magma mist to appear when debris falls into magma, which is a quite old and well-establish feature that any regular player would know about. Now, magma mist suddenly appearing after a 1 Z-level fall of a single stone onto dirt, which in my experience has never even caused a cave-in in the first place, is strange. How do you "Cave"-in a single tile of wall on the surface, anyway, unless you're horribly bad at taking things apart.
I do apologize for any such insult I may have thrust upon you, I guess I have a habit of taking out my frustration on others at times. Had some of that Viking's Fjord I had in my fridge, good Scandinavian vodka that, so I'm no longer phased by my epically-shitty week. Good moods for all, and to all a good night.
In all seriousness though, I don't mind the magma mist. I just hope the faux-magmist doesn't cause actual magma-mist-like damage, such as skin peeling and organ-boiling, because that would epically suck.
I guess in some macabre way Armok answered my call, because that's one less annoying Dwarven baby to pester me. Anyway, time to finish this succession fort, I'm just about done with my turn!