The bug

Pressing F3 + T does not reload advancements but does reload loot tables despite them being both in the data folder of a world.

Note: This might affect recipes as well.

How to reproduce

1. Open a world
2. Create the following folder structure in the data folder of the world
   • loot_tables
     • custom
       • test.json (attached file)
3. Create a chest with the loot table custom:test

   /setblock ~ ~ ~ chest default replace {LootTable:"custom:test"}
   

4. Open the chest
   → It contains a stone item
5. Change the test.json file (with a text editor) to give minecraft:dirt as loot
6. Repeat steps 3 and 4
   → The chest contains still a stone item
7. Press F3 + T
8. Repeat steps 3 and 4
   → The chest contains a dirt item
9. Create the following folder structure in the data folder of the world
   • advancements
     • custom
       • root.json (attached file)
10. Press F3 + T
11. Walk around
12. Open the advancement view
    → You will not see the custom advancement (beacon icon)
13. Reopen the world
14. Open the advancement view
    → Now you will see the custom advancement