I am pretty sure this has to do with world hopping, but often times if you visit the shark kitten den out of monsoon season in shipwrecked, you will find that even though the shark kitten den is just a pile of sand, shark kittens will still spawn from it.
Steps to Reproduce
Not sure how to reproduce this, but I am pretty sure it has to do with world hopping into shipwrecked from the other DLC's. The game appears to be confused and will allow the shark kittens to spawn by the den even when it's not monsoon season and the den is not even active.
This issue has not been confirmed by a developer yet.