There are some battles(not all) where once i kill Ares after he has used onslaught (Yellow), the onslaught countdown tile remains and will damage my character when the countdown is complete. Not sure if it has to do with stunning with him with Spiderman or not. I will check next time i play to give further information.
I have noticed a bug like this. If Ares kills himself with Sunder, the token will remain even with him dead. If not destroyed, it will damage my team but not heal the other team. If I use an AoE ability (Thor Green) the token will be destroyed.
https://forums.505go.com/discussion/2130
No fix yet 
My bad on the skill name. It is Sunder, not onslaught.