Okay, so I've played with it some more. It seems that the cart will only move if there are no blocks diagonally adjacent to it. See "Ex 1", when the cart is like that I and I hit the button, it will launch. However, if there are blocks diagonally adjacent to it, when you power the rails, the cart won't launch. See "Ex 2" and "Ex 3". Before the 1.9 update it didn't matter if there were blocks adjacent.
Why did this get resolved as invalid and changed to 1.8.4? Our Realm was automatically upgraded to 1.9 and this is a legitimate bug that came with it.
I did some more testing and I was wrong, it's not the diagonnally adjacent blocks that are causing it. I have other tracks with nothing around the start point that won't launch the cart for whatever reason.