Updated description by GoldenHelmet June 1, 2020

Steps to reproduce

(These steps are automated in items-stuck-in-a-filter.mcworld.)

1. Put 1 stackable item in each slot of a hopper/hopper-minecart.
2. Drop a different item on the hopper/hopper-minecart.
3. Drop an item that matches an item from step 1 on the hopper/hopper-minecart.

Expected result

The hopper/hopper-minecart collects/sucks the matching item.

Actual result

The hopper/hopper-minecart will not collect/suck the matching item until the non-matching item is removed.

Note: test analysis provided in this comment.
Code analysis corroborating testing in this comment.

Original summary
Hoppers not picking from multiple item types
Original description
When item sorting hoppers (as shown in the screenshot to sort redstone) has multiple item types (with the selected item type included) pass above it, the hopper will occasionally fail to suck in the selected item type from the group.

Steps to Reproduce:

1. Construct a single item sorter with a water stream as depicted in the screenshot with the top hopper set to sort an item type
2. Confirm the top hopper only accepts the chosen item type
3. Block the water steam
4. Drop several of the chosen item type
5. Drop several of another item type
6. Unblock the water stream

Observed Results:

All items (included the chosen item type) pass over the hopper.

Expected Results:

Hopper sucks in the selected item type and the rest flow by.

This does happen most of the time

Screenshots/Videos Attached: Yes

Video of occurrence

Device: Asus ZenFone AR