Comparator won't read contents of copper chest if a block is above the chest

Trying to prevent Copper Golem from being able to open this chest (see picture) when it is empty. I used this set up multiple levels below this with trapdoors in place of the block over the chest, and another block with dust on top in place of this piston to activate and de-activate the trapdoor. For whatever reason, this will not stop this Golem from looking in the chest. I have been trying other methods, and when the piston gets a block update, this method does work; but for some reason, whenever items enter the chest, it does not update the comparator (and as such, the piston does not move out of the way).

Just about at the end of my tether trying to puzzle this through, suggestions welcome.

Might also be worth saying this is a Realm. Usually has no impact whatsoever on Redstoning, but this is really a head-scratcher.