The bug

When detector rails are placed beside powered rails on the flat, as a minecart travels over the detector rail the powered rails receive power long enough to boost the cart.
This mechanism gives nice clean booster sections on long tracks.

However, the same structure replicated on a slope doesn't behave the same way. As per the screenshot, if a detector rail is placed on a slop with a powered rail above, while the detector rail does function (as per second screenshot), it doesn't supply power to the adjacent powered rail.

I imagine this has something to do with the powered rail being on a block above the detector rail, however, the rails do join diagonally and it would make sense for the power supplied by the detector rail to also flow to adjacent diagonal powered rails.

Edit: Detector rails placed before powered rails when heading down a slope function correctly, again, due to how redstone functions.