My intention was to create a simple trapdoor using a sticky piston, trapwire and a NOT-gate. However, the piston doesn't seem to update its state and doesn't retract, unless I destroy a block directly next to it.
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Can confirm...

You've created a piston BUD - A bug that Mojang won't fix because youtubers love it.
See MC-108

You can make your trapdoor work using a repeater, to avoid powering the block above causing the piston not to update. See my attached screenshot.