Okay, strictly speaking, the miasma is totally real, it's the rotting corpses that aren't quite there.
The story begins when a couple of squads out of a goblin siege find themselves in Cerolemal's finest drowning corridor, with predictable results. Now even through the fortress is nearing its 20th birthday, I've never gotten around to setting up power for the pumps that un-flood the corridor. For the same reason that there isn't power, it's a rather long walk from whatever dining hall my usually-idle pump operators are chilling in. (Normally their only job is periodically filling the basin under the magma piston.)
Anyway, since the victims weren't carrying enough iron to worry much about preserving it, I decided to use some bridges to atomsmash the water, and fluid flow naturally pushed some of the corpses under the bridges in the process. Totally expected. Several weeks pass, the floor is all blue 1s and 2s, and right on schedule I have miasma from the un-smashed bodies. (There's garbage compactors at work, of course I'm not going to let the haulers in yet!) Except then I notice it: I also have bright purple miasma tiles spawning on totally empty floor spaces as well!
So I'm thinking that here I have some further evidence for the popular theory that atomsmashing doesn't destroy objects quite as completely as advertised. I don't have an appropriately-timed save to compare to, but just eyeballing I wouldn't be surprised if those miasma spawning tiles correspond to the locations where the former owners of those obliterated corpses met their watery end.
As long as they also stop spewing miasma on schedule, this isn't actually a problem. But is this a phenomenon that anyone else has seen?