summary:
pistons should retract pulling slime blocks with them

What I expected to happen was...:
a piston on top of another
the pistons should retract pulling the upwards facing piston and the slime blocks with it

What actually happened was...:
all the powered pistons retracted without pulling the upwards facing piston or the slime blocks with it

Steps to Reproduce:
1. place 2 pistons on top of each other
2. place 3 slime blocks in front of the top piston
3. place an upwards facing piston under the slime block in the middle
4. power it