When i use a piston faced down and put a block of redstone 2 blocks on top of the piston, the piston activates and stay activated forever even when removing the redstone block.

Happens in creative and survival at least

What I expected to happen was...:
Nothing happens, the piston stays in his initial form

What actually happened was...:
The piston activates and stays activaated forever

Steps to Reproduce:
1. Put a piston face down
2. put a block 2 blocks up on top of the piston
3. the piston activates even with no contact with the block
4. remove the redstone block
5. the piston does not change his state