Mac OS X
When attempting to replace any vanilla assets with the "minecraft" data namespace, the game doesn't ignore the .DS_store files, which Apple uses to store file display info, and which users cannot edit or see.
This causes an error, but for some reason only occurs when attempting to replace any of minecraft's vanilla data. (recipes, advancements, loot tables.) (Not tags.)
The error is not displayed when using folders to edit data packs, but it is when you use a .zip file.
You can test this out by using the attached data pack, which attempts to replace the 'Postmortal' advancement's display title with "Butts".
This doesn't affect the new 'tags' system.
This does affect advancements, recipes, and loot tables.
This report should be used instead of
MC-121379, as MC-121379 has some differences from this issue, mainly that the game attempts to read the files as a different type, cutting off some of the lettering, and this does not occur in this error. MC-123516 should be marked as a duplicate of this issue, as it is just talking about advancements, when all the data 'things' (except for tags) are affected.
This only affects Apple devices.