Inconsistency with pistons pushing items

What I expect:
Consistency when a piston pushes a block with things attached to that block

What actually happens:
Sometimes the thing attached fall off (even through they can attach to the next block) and sometimes they stay attached (if they can attach to it)

Steps to reproduce:
This is strange as it only occurs randomly but

1.Put some jungle logs
2.Put cocoa beans on them
3.Use a piston to push the logs
4.Repeat until you find inconsistency in your result