Steps to reproduce:

• Look at the included files
• Note how it includes tons of duplicates that change nothing from vanilla

Alternatively, for an in-game effect:

• Create a datapack that changes something unrelated, like a damage type
• Add that datapack with the update_1_21 pack above it and vanilla below it
• Note how the changes from your datapack do not take effect at all.

Expected outcome:

• The update_1_21 pack contains only files that are actually different from vanilla.