I know why now. If you mean Rio are this one
View attachment 330196
You know how her ultimate skill kinda OP (hit every enemy in right situation) ? Well, she got some kinda nerf so you can't used her in dungeon run willy nilly. Each time a battle start, she damaged herself for certain amount (someone said about 15% max health ?)
Now, did you know dmg multiplier ALSO multiply this self damage ? So after certain multiplier she always kill herself before battle start, because that damage based on percentage of her max HP and then multiplied.