When cart dispenser has reinforced booster track in front of it, the cart is dispensed and then is immediately going forward, the same is with wooden booster track. However when there is normal booster track dispensed cart just sits there.
Setup:

Result of putting cart on detector rail:

I have no control over the behavior of vanilla Powered Tracks. For situations like this, I recommend Control Tracks.

Similar setup, but with normal dispenser works without problems in vanilla. Dispenser and cart dispenser also have different redstone behavior and with RC you can't use normal dispensers to put carts on tracks.

Its because Cart Dispensers are not considered a "normal cube" due to the fact that the block is not opaque.

There are plenty of better tracks to use anyway.