The Action with sand, tin, copper, and so forth boxes was able to lay a 3x3 set of boxes around and on top of a box unloader. repeated experiment with empty boxes and all 3x3 were able to be placed. The Reaction the unloader took materials from the boxes in the 3x3 area. the loader was able to put the materials into the boxes in the 3x3 area - since boxes are 'incorporeal'(like most build-able items in the game), items can go through them with enough speed to reach the loader.

Why do I consider it a bug? I consider it a bug because if you are trying to save space by placing unloaders one above another facing the same direction, thus placing different boxes above each other, does not guarantee the unloader will grab from the correct adjacent box. This can lead to production problems if you are trying to minimize the amount of area you use for production. Equally, the issue also persists with loaders in a similar way. Two adjacent loaders facing the same direction with empty boxes around them will not necessarily direct materials to the desired boxes.

I tried different patterns for the boxes, and the only conclusion I could come to was that the priority for loading/unloading from boxes is not a 'always takes from/puts in box X first' pattern.