Phantom redstone signal activates pistons.
How to repeat a bug:
1) put the piston directed down;
2) Place a block on the piston and a redstone block on the block;
3) break the block then redstone block;
Done.
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Hello! This looks to be a duplicate of MC-108, which has been marked as being intentional by the developers. Feel free to take a look at that report for more information.

dear mojang i bought minecraft using microslof acount but when its their its says demo in ful version i littery spend my mone y please help me with this isuee
The bug tracker is unable to provide you with assistance so please instead contact Mojang Support.