It happens to children nodes that generated by gui.new_box_node()
I don’t know whether it’s a bug or I missed setting something on children nodes.
local node = gui.get_node("box")
local child_node = gui.new_box_node(vmath.vector3(), vmath.vector3(50))
gui.set_parent(child_node, node)
gui.set_alpha(node, 0.5)