Item CustomModels do not render

When attempting to use CustomModelData for items provided by a datapack, the items do not display their custom models. See the attached screenshot for an example of the bug.

Steps

1. Create a new world.
2. Install the datapack attached below (datapack taken from attachment on https://bugs.mojang.com/browse/MC-138892).
3. Use the following command to give yourself a water bucket with CustomModelData:

   /give @s minecraft:water_bucket{CustomModelData:1}

   The item that appears in your inventory will maintain the regular water bucket texture.