Minecarts in survival mode will sometimes drive off an edge into a one wide gap and sometimes won't, it appears to be random and unpredictable. Increasing the speed of the minecarts and the block type that forms the gap seems to have little effect. Even when blocks with a smaller hitbox such as trapdoors (see screenshot) are used to form the gap, the issue remains.

This is particularly annoying as when placing villagers in minecarts in adjacent slots a drop is essential since the villagers can drive the carts back up slopes or flat sections of track.