Hoppers appear to randomly stop draining items through shelves

Steps:

1. Place a shelf with a hopper below and a hopper pointing into the side or top

2. Fill all three shelf slots with a full stack of blocks (same blocks)

3. Fill the input hopper with stacks of the same blocks

At some point, the bottom hopper stops filling until you clear a slot, it will resume filling but again stop when you return items to the slot.

Attached is a video capture of the problem

I have been able to replicate this issue three times so far but it is somewhat random