This isn't necessarily a bug, but possibly an inconsistency that could be looked at. I've had to help 3 players with this problem, and it's related to the green fog mist spawn.
The green fog has a habit of turning people into frogs on a successful attack, but they revert back to normal upon being downed. However, the 100% spell failure that comes from being hexed remains, even if they get back up.
This leaves people confused as there's no visual indication that they are still cursed. I've managed to get rid of the spell failure by using remove curse on people with this problem. I think it would be less confusing to players if they either stay as frogs when downed, or the silence effect is removed when the polymorph effect is.