Steps to reproduce

1. Place 2-6 hoppers pointing downward in a horizontal square or rectangle orientation as shown in the attached screenshots.
2. Place another hopper such that it points into the space above one of the hoppers placed in step (1).
3. Place a chest boat or chest minecart so that it straddles the hoppers placed in step (1).
4. Put a stack of items in the hopper that was placed in step (2) and wait for the hopper to push out all of the items.
5. Check how many items are in each of the hoppers placed in step (1).

Expected result

About the same number of items end up in each of the hoppers placed in step (1). In other words, the items get distributed randomly because the hoppers update in random order.

Observed result

The hopper directly underneath the spot where the upper hopper (placed in step (2)) pushes into the storage entity gets fewer items. With two hoppers underneath the items distribute in a 2:1 ratio, with four hoppers underneath the items distribute in a 5:5:5:1 ratio.

When using a chest boat on top of hoppers, the items do not distribute evenly to all hoppers when the chestboat is being fed by a hopper. Whichever hopper is below the one that feeds the chest boat will receive few to zero items. This occurs whether the feeding hopper is above the chest boat or feeding into a side. This was tested in multiple configurations of a chest boat sitting on top of four hoppers, with one hopper feeding items into it.