I was on project when I found this bug.
First I've got a row of hopper who are pointed on the east side ,on an other hooper.
In the second row i've got hopper who point on an other row of hooper below.
Finally I've got a third row of hopper who point on some chest.
When I lock the second row some item drop in the hopper below even if it is lock.
That happen on some position because I can't reproduce it everywhere, so I will give you my map.
In the map you will need only to put items on the first hopper and can lock the others to see the bug on the third one.