Falling sand drops only in North direction

What I expected to happen was...:
Items are evenly spread because of random item momentum

What actually happened was...:
Items only had north momentum and not south momentum. East/West was not affected

Steps to Reproduce:
1. place a piston facing up with multiple sand blocks on top of it
2. activate and deactivate the piston quickly (possibly with an observer clock)
3. observe that sand items do not land on the south side of the piston

Notes:

The issue is not locational–it behaves the same in positive and negative coordinates 

It does not affect manual block breaking