Pistons don't update when given a there power source is removed by slime blocks

When having a downward facing piston you can place a redstone block on top of it and a slime block on top of the redstone block. If you move the slime block you will move the redstone block which should cause a block update. Instead it causes the piston to retract and then extend again and you can move the slime block as far away as you want and the piston will stay extended until given a block update.