The Bug

Additional/custom enchantments from data pack are ignored when uploading a data pack to a Realm in Java Edition 1.21. This causes the enchantment to no longer appear on books in the Creative inventory, prevents use of enchanting table, and removes all enchantments from all newly generated loot including fishing treasure, chests, and entity drops.

How to Reproduce

1. Download a data pack with custom enchantments, for example the attached data pack
2. Add this data pack to your Realm in 1.21
3. In-game, attempt to acquire the custom enchantment book in the Creative inventory, attempt to enchant any normally enchantable item in an enchanting table, or generate loot that includes enchanted books/equipment
4. Notice that the custom enchantment doesn't appear on any enchanted books in the Creative inventory, items cannot be enchanted in the enchanting table, and generated loot is missing enchantments (e.g. enchanted book with no enchantments listed)
5. Now try to add the same data pack to a singleplayer world in 1.21
6. Notice that the custom enchantment appears as an enchanted book in the creative inventory, enchanting tables work as normal, and newly generated enchanted loot includes enchantments as intended