In my setup I load a villager onto a minecart then drop the minecart down a shoot onto powered rail that is powered. Half of the time the minecart will take off, in a random direction, the other half of the time it just sits there.

The same issue occurs when dispensing a minecart directly onto powered rail that has a powered rail which is powered, half the time it will take off, the other half of the time it just sits there.

Minecarts should ALWAYS move in a random direction when placed on powered rail that is powered.