Dolphins don't spawn in cold biomes/oceans

The bug

Dolphins don't seem to be spawning in cold oceans anymore.

How to reproduce

Create a new map with a random seed, plug the seed into mineatlas.com to find oceans to teleport to. While in "cold" or "deep_cold" ocean, move around a bit in creative mode to get water mobs spawning, then run:

execute as @e[type=dolphin,distance=..200] run say hi

If there are dolphins within 200 blocks of you, you'll see them talking in chat.

In every other ocean type (except frozen) I get between 5 and 10 dolphins saying hi, nothing in cold oceans anymore.