When using the Minecart Improvements Experiments, minecarts can stack into each other when falling vertically into a rail, even though this cannot be achieved by a player when placing them into a rail individually.

According to the changelog, this should no longer be possible:

Minecarts can no longer be placed within another Minecart by the player or a dispenser.

How to reproduce

1. Place a certain configuration of rails where you place a minecart and it falls into a rail from above, see the attachments.
2. Place the minecart in a way so it falls into the lower rail.
3. Place a few more.
4. Notice how they stack each other in the same position in the rail.

Expected result
Only one minecart would be placed into the rail, other minecarts trying to fall into the same position would either stack on top, not inside the former minecart. Or else minecarts that fall into the spot where there's already another minecart would break and drop as an item, similar to current dispenser behavior.

Observed result
Minecarts falling into a spot where another minecart is already placed can stack into each other resulting in old behavior, which contradicts the changelog and new minecart placing behavior.