That said, I don't remember tadpoles coming out of some other true souls either. Does Minthara have a tadpole when you kill her?
Gut and Ragzlin have tadpoles that try to escape, and if you're quick and accurate (or are sharp with jumping back into turn-based as soon as combat ends), you can kill their tadpoles as well.
Minthara does NOT have an escaping tadpole like the others, however, Minthara is also built and coded like a recruitable companion - when you take her gear she end sup stripped to her underwear, for example.