Observers do not detect shelf contents changing

Expected behavior:

When multiple powered shelves are next to each other, the shelve that has their contents changed should cause an update causing the observers to output 1 redstone tick.

Actual behavior:

Only the shelve that was clicked on causes an update for the observer.

Steps to reproduce:

1. Put down two or three shelves next to each other and power them with redstone from the back.

2. Put observers on the bottom of the shelves.

3. Have for example hotbar slot 1 and 2 filled, with 3, 4 and 5 empty, and right-click on the frame in the middle. Only the clicked on frame will send an update.