The bug
A repeater, comparator or observer will not update the block in front of it if that block is non-solid. This provides another way of quasi-powering pistons.

To mods
Even if MC-109266 is WAI this is still a bug as the piston actually receives power. If Mojang do not want power going through non-solid blocks they should not actually allow those blocks to be powered which would require fixing MC-108.