I've found plenty of dupes for hoppers beside detector rails not being powered, but nothing about the hopper above the chest being deactivated.

With a power rail under the chest, this example works okay, but if I change the power rail to a detector rail, the hopper stops dropping into the cart.

My plan is to use a comparator on the hopper & the chest and use a piston to push the cart onto power rail if either the hopper is empty or the chest is full.

The detector rail should only affect squares adjacent to it, right? Not the hopper above the rail?