The bug

When applying loot tables to containers, the contents of barrels cannot be modified.

How to reproduce

1. Apply a /data merge command to a barrel to give it a loot table

   /data merge block ~ ~-1 ~ {LootTable:"minecraft:chests/buried_treasure"}
   

   → Contents will not be altered

2. Try using the /setblock command

   /setblock ~ ~ ~ minecraft:barrel{LootTable:"minecraft:chests/buried_treasure"}
   

   → The barrel will not have the loot table contents

All commands used worked properly on chests and shulker boxes but not barrels.