Other bug report was misunderstood, please read carefully!
Double piston extenders no longer retract both pistons at the same time, when they are depowered in the same game tick.
This behaviour was classified as "Works As Intended" in MC-9342. People were used to this behaviour and many redstone contraptions rely on the sharper timings such a behaviour enables. Removing this feature would in some cases require major adjustments to those. This was removed in the 1.9 snapshots.
How to recreate:
This behaviour was always dependant on the update order. The piston in the back had to receive the update first. To influence the update order one way was to power the front piston with a comparator and the back piston with a repeater. This behaviour was also reliable with doublepiston extenders facing upwards, being de-powered by a torch or a minecart powering a downwards facing double piston.