When I'm trying to make a redstone contraption with pistons, whenever I place a block (anything, from unpowered redstone to spruce logs) a block away from the piston (It also sometimes happen a block or two away) the piston receives a fast redstone signal wich activates it and then quickly deactivates it.
How to replicate:
Place a piston on the floor facing to the sky (I only have tried with sticky pistons), then place a slime block on top of it and then place a redstone block on top of the slime block. Nothing happens.
Then place any block next to it and you'll see that it moves quickly.
Screenshots below.
This isn't like a redstone signal can power the piston diagonally or a block away, it's a non-redstone related block, the block that gave the piston a signal was a green concrete block, and the piston retracted immediately after it received the supposed signal, even when the concrete was still there.