When you are creating a custom enchantment through a datapack, normally the custom enchantments appear in the creative inventory. But if in the field that says "supported_item" you use an item tag that is custom from your datapack the enchanted book won't appear in the creative inventory. The enchantment will still work as intended.
How to replicate:
Create an item tag inside a datapack I placed this in: data\minecraft\tags\items\enchantable\
[media]
Create two enchantments, one that uses the supported_items field with minecraft tags and one that uses the custom tag, these are the ones I used which should work the same way as multishot:
and
[media]I placed them in:
data\minecraft\enchantment\...
Open a world with the datapack in creative and in the creative search bar write multishot. Only the visible one appears.
But if you pick a bow (because the one that uses the custom tag includes bows as supported items) and try the command "/enchant @s minecraft:multishot_not_visible" on it, it will work as intended.
The only problem is that it doesn't appear with custom supported items tags in the creative inventory.
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We're tracking this issue in MC-271634, so this ticket is being resolved and linked as a duplicate.
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