I know this would be a bit counter to the way standard D&D is designed, but I feel like this is a plot hole that would be solved by saying the party members are just incapacitated in some way, not dead, so that it makes sense why they can be revived so easily and why the tadpoles stick around.
Maybe they can introduce a permadeath mode to the game where your party members actually die when they're killed and you see the little tadpole crawling away from the corpse. XD
It would not be too difficult to invent a reason why the tadpole prevents "real soul-death" and could linger on for a long while in a mangled coprse to nable a kind of pseudo-resurrection.
Like the tadpole can change his host into a tardigrade husk.
But what seems far more difficult is to make this happen only to the PC"s, whereas all the other (non-IC) true souls lose their tadpole a short while after being slain.
The ease of resurrection also conflicts in a major way with the storyline of the necromancer's book of Thay. This powerful red wizard struggled hard with arcane magics and never succeeded to resurrect a close one, whereas all the PC's just start the game with a perfectly working resurrection scroll.
