I am unable to open a worksite's interface while holding a block if that block cannot be placed adjacent to the worksite block. On successfully opening a worksite's GUI with a block in my hand, the block is visually placed adjacent to the face I clicked on and also momentarily subtracted from my inventory. Upon leaving the worksite's GUI, the placed block is no longer there and is returned to my inventory. The same thing happens with placable items such as torches.

This behaviour is peculiar to Ancient Warfare worksites. Vanilla chests/furnaces/etc are not affected.