Minecarts with chests don't appear in the generation of abandoned mines!
Steps to reproduce:
1. Create a world with the seed 15214.
2. Go to coordinates X=2856, Y=31, Z=63 to teleport to the mineshaft. After searching around for a while, you should be able to find some empty rails like the one in the screenshot that SHOULD have minecarts with chests, but don't.