according to my observation, pistons, sticky pistons, dispensers and droppers can be activated by applying Redstone signal to the space above it; so if there's a Redstone block/charged block/Redstone torch etc two block above it with air in between, the (sticky)piston/dispenser/dropper will activate remotely and deactivate once the signal is removed, but only when the block adjacent to it is changed.
I don't know a lot about the technical stuff here but I think this is due to some 'update'(possibly block or chunk update), which is triggered by the block space adjacent to the piston/dispenser/dropper changing(placing/destroying etc.), which, combined with space above's 'conduction' caused the remotely-activating-and-non-retracting piston; the dispenser/dropper are impulse-type blocks, so they only show remote activation.
in the examples below, most blocks activated by placement of a signal source adjacent to the air above said block/'updating' said block by destroying block next to it after placing signal source, and deactivate by 'updating' the block (by placing blocks next to it) after removing signal source. most signal sources are block, so their placement themselves will bring the 'update'(see the text example below); the button is a special case, as it doesn't have any signal on placement, so I used a second piston to simultaneously change the space next to target piston(in this case, place a piston head in it), thus 'updating' it while giving power to the air above it, so it activated as expected.
these two bugs/unintended features((sticky)pistons/dispensers/droppers powered through air above & needs 'update' to activate/deactivate) can explain a lot of issues here(such as placing a redstone block on top of top-facing piston causing the piston not retracting; removing the redstone block give the piston 'update'(because the block is adjacent to its head) so it will retract.):
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