I actually have seen that and specifically the water or static cloud they seem to remain indefinitely unless they are triggered a number of times.

However in the case of the frogs I bet that the environmental effect are cursed and therefore more potent and eternal. But you can bless them to remove the curse.
It adds another layer to the environmental hazards which I find pretty deadly and rather cool.

All your character will be able to cast bless so I am fine with this.