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
[media]
In the images named "image-2021-08-8(4 through 8)", I took a screenshot in the exact same location (240,40,176) in seed 123. I did a total of 8 tests & found anywhere from 0 to 5 chest minecarts in the entire mineshaft. Below are the coordinates of the rail beneath the chest minecarts in each attempt:
Run 1:
256,36,168
283,36,172
Run 2:
242,36,156
199,31,128
Run 3:
199,31,123
176,35,119
Run 4:
186,35,119
Run 5:
247,35,149
225,37,140
224,36,130
252,40,154
283,38,150
Run 6:
268,36,166
280,37,124
Run 7:
none
Run 8:
224,37,148
215,36,148
In each of these runs, not a single location was chosen in another run to generate the chest minecart.