water_animal spawn rule seemingly never spawns custom mobs

Partner Rank: 3

Verification builds: 1.11, 1.12

Summary: I am completely unable to spawn custom mobs under the water_animall spawn cap. animal and monster work fine. In slack, I have posted a world in which a behavior pack disables all vanilla spawns, and tries to enable a custom mob to spawn underwater (using the exact same spawn rules vanilla cod use). However it does not spawn.

Repro Steps:

1. Download the world from Slack.
2. Open the world and fly around or underwater; nothing should be spawning.
3. Check out the behavior pack, which has disabled all spawning save for a single custom entity.

Expected Results: The custom entity should be spawning alone underwater.

Observed Results: Nothing spawns. If the spawn rule identifier is changed back to minecraft:cod, and the disable-cod file is removed, vanilla cod spawn.

Screenshots/Videos attached: No

Notes:
It is very possible I am just misunderstanding here, but after thorough testing I was easily able to use the other two mob caps, but completely unable to use this one for anything other than vanilla mobs, even when my custom mob is the only potential spawn option.