This is a major bug in MCPE Pistons.
It seems like pistons, or maybe even redstone generally move slower than PC. For example the fastest double piston extender in PC takes 6 ticks to extend and retract, it uses two repeaters. One repeater is at 2 ticks, the other is at 4. On PE the same design can be build, But it needs to be twice as slow. 4 ticks and 8 ticks. If you go lower than that timing, it won't work. This applies everywhere in pistons, not only in extenders.