When a piston get activated directly (so direct from a repeater or redstone dust) and a platue is placed above the incoming signal and top of that is a redstone signal which goes directly above the piston(it doesn't have to touch blocks like in the screenshots) and the power source to the piston is removed but there is a power source for the upper redstone circuit then the piston stays activated and doesn't accept block updates untill the redstone (from the upper circuit that was diagonally above the piston) is turned off
the activated redstone part can be redstone dust and a redstone repeater both on the upper and the lower redstone circuit
happens with both normal and sticky pistons
summarized: pistons which get activated and have a redstone signal diagonally above them don't get any block updates unless the upper redstone signal is gone
(has not been tested if the same happens from underneath the piston)