The cool down timer of hoppers is used for picking up and pushing out items.
This leads to hoppers that push out items, to only pick up items every 8th game tick.
If items happen to cross the hopper only outside of these ticks, the hopper might never pick up items, even though there are free slots in it.
What I expected to happen was the hopper picking up items.
What actually happened was the hopper not picking up items in certain cases.
Ways to reproduce
With hoppers
(by @unknown)
[media]there's 4 hoppers in a loop on the top, and you should first power the block then put one item in each set of looping hoppers
With hopper minecart
[media]The minecart is driving back and forth on a 9 block long track.
In the center there is a hopper below the rail with some items in it pointing into another hopper that points back (pushing items back and forth between each other).
Depending on when the minecart started driving, the hopper will not pick up items from it.
Note that this is only the case when the hopper is pushing out items. If you break the second hopper it would pick up items fine.
The test setup is attached as structure file.
[media]
Confirmed in 1.13-pre8. Added a picture showing another example of how to reproduce.
[media]4 hoppers above forming a loop with 1 item over the green block. 2 hoppers below pointing into each other with a different item next to the green block. Turn the switch off to unlock the starting hoppers and the lower hopper will never pull the item out of the upper hopper.
EDIT: Build this starting with the green block, then power the block with the lever so that the hoppers are locked when it is built.