If a piston is unable to push but is still powered, the piston doesn't remember that it is being 'powered'. so when an additional redstone signal is given through a block (i.e. another redstone line powers the piston through a block) the piston will update itself, despite the fact the the piston was already 'powered' (even though it was unable to push).
How To Replicate:
Place a block with two levers attached. Place a piston next to that block facing away. Place another block in front of the piston. Lastly, place a furnace in front of the last block and THEN flick one of the levers.
Your setup should look like this:
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Destroy the furnace. The piston should stay retracted.
What i expected: flicking the other lever should NOT update the piston as it is 2 blocks away and the piston is already powered by the first lever.
What happened: flicking the other lever results in the piston updating and extending.
The piston should still be in a 'powered' state, but yet it still re-powers. even whan that source is from 2 blocks away.
by flicking the 2. lever youre providing another blockupdate to the piston wich indeed tells it its powered and therefor will extend. completly intended and fine behaviour