This happened to me twice in a row just now, so I'm pretty sure it's a bug. I had a mutant out (one time it was a time nymph I think and the other was a minor vampire), the AI poisoned it, I TU'd it, and the mutant came out being the same in every way.
As far as I know, TU'ing a mutant is supposed to/usually increase the stats and usually changes the ability.