This is independent of the Minecart Experiments being enabled, although with them disabled many more minecarts are left behind.

Steps to reproduce:

1. Build the setup shown in the attached video.
2. Summon a lot of minecarts on one rail (minecart cluster).
3. Push them in the direction of the soul sand.
4. Some of the minecarts (three in most cases with the experiment enabled) are left behind on the second soul sand block, but not on any other one - the minecart cluster will be unaffected by the other soul sand blocks in the setup.