Using the same seed and going to the same chest in the same structure will yield different loot each time you load a new world with that same seed.

2 ways to reproduce:

1. Create a world with a Bonus Chest
2. Check chest, note loot
3. Exit world, recreate world
4. Check chest in this new but identical world
5. Loot is different

1. Create a world
2. /locate Mineshaft
3. Look for a minecart with chest
4. Note coordinates
5. Check loot, note loot
6. Exit world -> Recreate world
7. TP to coords of minecart with chest
8. Check loot, there's different loot

EDIT: It turns out that the loot chests' loot table seed is being set, but it's not being set depending on the world seed. I found this out with /data get block <coords>.

EDIT 2: I know that this is not WAI because if you read this, you will see that the data structure block sets the loot table seed dependent on the world seed. This was also the behavior of chests in structures prior to the introduction of loot tables, and I see no reason why they would set the loot table seed in new structures, but not old ones that don't use structure blocks.