Setup as in image [2018-08-02_00.57.19.png]

As the hopper minecart goes over the detector rail with a stack of planks inside of it, the comparator sends a signal to the "intersection" rail and the rail switches sides and switches back when the redstone signal dies out. This is normal.

Setup as in image [Bug.png]

As the hopper minecart goes over the detector rail with a stack of planks inside of it, the comparator sends a signal to the "intersection" rail, however, the rail does not switch sides. It switches sides only once the redstone signal is gone. So instead of the regular back and forth switch, it switches once and stays that way.