I go into the Debug world regularly. I noticed that just after the Shulker Boxes were added, they were all invisible. Their hitboxes appeared, but they were not visible.

This still affects the following versions:
minecraft: yellow_shulker_box, facing: south, west, east
minecraft: lime_shulker_box, facing: all directions
minecraft: pink_shulker_box, facing: down, up, north
minecraft: gray_shulker_box, facing: down, up
minecraft: white_shulker_box, facing: down