Vertical double piston extender causes player to phase through

Standing on a block which is pushed upward by a double piston extender will cause the player to "phase" through the block provided adequate blocking (otherwise the player get pushed aside). This has to happen quickly - under a second. Using full and half blocks will cause the player to phase under the block, whereas with stairs, the player will stand on it as if it was a slab (further note: player cannot crouch while in stair). 

Demonstration 1: https://youtu.be/TRPd-fd9lNg 

Demonstration 2: https://youtu.be/SJwipJGt32w