I'm not sure if this is a bug, or intended but as you can see from the picture, there is a rail on a furnace. The rail seems to be causing the slime block to stay in its place. It only happens if the rail is ontop of a non-pushable block. Otherwise the slimeblock breaks it.
MC-54794
Piston doesn't retract slime block
Resolved
Invalid
0
Yes, because the slime block sticks to the rail. It is intended. This can only be accomplished by pushing and pulling the rail horizontally. If done vertically it will not work. Also the rail will break if the non-movable block is not there, and doesn't have a surface to be pulled to.