Using setblock or fill on a piston head leaves an item drop

When using setblock on the piston extension of an extended piston, a piston item is dropped.

This also happens when using the fill command when pistons are facing certain ways(due to the nature of the fill command using multiple setblock commands)
When using a fill command, if an extended piston facing either north, west or down is present in the area, it will drop a piston item.

How to recreate the problem:

1. Place a command block with the command

   fill ~1 ~ ~1 ~5 ~ ~5 air
   

2. Place a piston in the area to be filled facing either north or west.
3. Place a redstone block next to the piston, making it extended.
4. Place a redstone block next to the command block.
   A piston item will now drop.