Hopper minecarts can't pick up items through container blocks

Hopper minecarts can pick up items that are on a normal block one block above the hopper minecart, but can't pick up items that are on a container block one block above the hopper minecart. The same thing happens if you put a hopper minecart partially on top of a hopper and put an item on the opposite side of the hopper - it won't pick up the item with the hopper minecart there.