This only realistically affects singleplayer. When you open a vault, you get a different set of rewards each time if you've created multiple copies of a world. This means for example, if you see a heavy core in an ominous vault, it's theoretically a guaranteed chance to get it simply by making copies of your world before opening the vault, repeating the process until you eventually get it.

Steps to reproduce:
1. Find a trial chamber and locate a vault.
2. Press F3+C to copy your coordinates.
3. Exit and make a copy of that world.
4. Open that vault in one of the copies of the world.
5. Open that same vault in the other copy of your world.

Observed results: I got a different set of rewards each time.
Expected results: I should get the same set of rewards each time.

Video demonstrating the issue: https://youtu.be/MFutDmMbWj8