It's not a plot hole. It's a very simple rule that should be obvious - tadpoles only leave bodies that cannot be revived.
Did you try using a revivify scroll on Edowin or the dissected drow? You can't. You also can't ask the undead guy at your camp to revive them.
So tadpoles only leave bodies when there is no chance for them to complete their task with that body. Otherwise they wait.
