If you create a datapack which edits enchantments and make the supported_items field be a custom tag you've made yourself, the enchanted book for that enchantment will not show up in the creative menu. The enchantment still works perfectly fine and can be applied to the correct items listed in the custom tag. 

This happens even when the tag is placed in the exact same enchantable folder that the other tags used in enchantments are located. The enchanted book will show up when the field is changed to a vanilla tag present in the enchantable folder, but using a custom tag in the same folder causes the issue.