Minecarts acting strange

Even when minecarts are facing down a hill they do not aways fall, even when a piston pushes them.

What I expected to happen was...:
That the minecart would go down the slope.

What actually happened was...:
The minecart got stuck facing downwards

Steps to Reproduce:
1. Make a piston pull down the block the minecart is on.
2. Make another piston which will push the minecart (with a repeater delay of 2)
3. Make track which goes downhill.

(I have included 5 screenshots showing different examples of this, all on the same map)