Pistons behave like half slabs/transparent blocks regarding redstone, this means that redstone dust stays connected even if it is "blocked" by a piston.

If redstone dust is powered from below a piston, then travels up to one block next to the piston nothing happens. If it travels another block, the piston extends. See the attached screenshots for details.

I expected the piston to react the same in both situations, either extend or retract.

This is a duplicate of MC-80848, but since that was wrongly closed, and bug reports unfortunately have a very small chance to get re-opened, I created this one.