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.
That is a fact, yes. But according to which logic does one know, if a dead body can be resurrected?
Is it the time passing? Edowin died in front of us, so time seems not to be the issue.
We can resurrect fallen companions even after days (or at least after a couple long rests. How much time really passed is impossible to say, because the game does not keep track of time). So time of death has nothing to do with it.
Is it how someone dies, to which injuries, poisons or diseases?
My companions died to all sorts of physical, elemental & magical injuries. The tadpole did not mind, obviously. And Edowin died due to wounds he got from an owl bear. So nothing "unusual".
So if it is no plothole, then what logic explains, how the tadpole knows the body can be revived or not?