I have not been able to confirm if this was the case at the time of implementation, but when you changed the specs so that the hopper could suck items that were more than 1/2 block and less than 1 block, the hopper could suck items even if the hitbox was 1 block high at a mud block.
Also, Soul Sands has a hitbox that is 1 block high, but the hopper can still suck items!

Observed Result

• The hopper sucks in items on a block that is 1 block high in the hitbox.

Expected Result

• Perhaps the hitbox display should be changed to show less than 1 block.
  Or maybe Mud Blocks and Soul Sands could have been made as blocks where items could not be sucked into the hopper.