The bug

The generation of chest minecarts in mineshafts seems to not be tied to the world seed, unlike nearly all other generation features. This means that if you recreate a world that had a chest minecart in a mineshaft with an identical seed, the new world simply might not have it.

How to reproduce

1. Create a world
2. Locate an abandoned mineshaft, and find a chest minecart (I can't give you a seed/coords for this because of this very issue!)
3. Note the coordinates
4. Create a new world with the same seed
5. Teleport to those coordinates
→ It's probably not there