The bug

When you attempt to use a fully valid data pack, with a function tag set, it completely works in folder form. However, the second you zip it, it states that the function is not recognized.

How to reproduce

1. Download the tick_tag_test.zip data pack below and place it into your world
2. Type /reload while in your world
3. Observe that the console says "woo", as it is running function testing:tick correctly
4. Now, zip the data pack, and then type /reload again
   → See that the data pack does not work, and sends the error:

   Couldn't read function tag list minecraft:tick from minecraft:tags/functions/tick.json in data pack tick_tag_test.zip
   com.google.gson.JsonParseException: Unknown value 'testing:tick'
   	at uv$a.a(SourceFile:144)
   	at uw.a(SourceFile:104)
   	at of.a(SourceFile:148)
   	at te.c(SourceFile:132)
   	at te.a(SourceFile:115)
   	at net.minecraft.server.MinecraftServer.a(SourceFile:1334)
   	at net.minecraft.server.MinecraftServer.a(SourceFile:346)
   	at cmd.a(SourceFile:81)
   	at cmd.d(SourceFile:128)
   	at net.minecraft.server.MinecraftServer.run(SourceFile:479)
   	at java.lang.Thread.run(Thread.java:745)
   

This is likely caused by the game recognizing the vanilla files before the custom ones, as this is the same case for vanilla advancements which have had their parents changed to custom advancements: It simply doesn't see the custom files and says it has an invalid parent, even though the parent valid and in the files.