Baby dolphins do not spawn naturally

The Bug

Baby dolphins seem unable to spawn naturally, unlike the other baby mobs added in the same snapshot as baby dolphins (squids and glow squids), making them unobtainable in Survival mode.

Steps to Reproduce

1. Download this world, extract the folder, and load it in the latest snapshot.
2. Press the "Kill Squids" button a few times to allow more dolphins to spawn.
3. Press the "Teleport Dolphins" button. Look closely behind you at each dolphin, noting if it is a baby or an adult.
4. Once done, press "Kill Dolphins", and repeat steps 2-4 several times, preferably for a few minutes.

Observed Results

Baby dolphins do not spawn naturally.

Expected Results

Baby dolphins would be able to spawn naturally.