I did a quick search but did not find anything in the Bug category that already listed this issue.
Basically the gist of the issue is that any hero that has an epic does not seem to apply status effects from their basic attack. Before they have their epic it works, then after the epic, it stops working. e.g. Stone Fist never chills on a melee attack once he has his epic.
From I've tested, it seems so. If you bring Rocky and Lilly and make a Green hero with an Epic a plant, they won't Poison on basic attacks. If you bring Zomm, Executum, and Brom on a team, Brom won't Disease on basic attacks. Also, I've never seen Squinch burn an enemy on Crit since he got his Epic.
There area few other comments related to this in this
thread. I am sure there are more that people will link in this thread.