This is occurring for me as well:
It seems random on whether a special will trigger poison, but only had it occur for attack-type specials so far and occurs several times per battle. Had it occur for blue, green and red characters in the attached screenshots.
Took forever to create an account on this forum and figure out how to creat a post, so dont have exact time, but occurred Feb 25 around 2:20-ish PST. Played S2-Stage 3 - tried each of the 3 mist levels twice, and bug occurred consistently in every battle.