Given the following filepaths:

Advancement: /world/data/advancements/custom/test.json
Loot table: /world/data/loot_tables/custom/test.json

The following are the corresponding resource locations, where loot tables display the expected behavior (the root folder name "custom" becomes the namespace) while the advancement is placed under the "minecraft" namespace:

Advancement: minecraft:custom/test
Loot table: custom:test

When both should be custom:test instead.