Redstone: Observer clock powers block over single block air gap from above

Some blocks are powered over a single block air gap by an Observer clock (two Oberservers facing each other) from above. This behaviour does not occur when the Observer clock is below or besides the block.

I have tested this with some blocks. These blocks are affected:

Dropper, Dispenser, both Pistons

These blocks are unaffected:

Trap Doors, Fence Gates

The attached image shows an example. The middle Dropper should have dropped iron ingots if this error would occur from the side.