Well, the White Box is below your text layer, so its 'above' it in the rendering order.
So, there are 2 ways I know of to fix this:
1 - move the White Box above your text in the hierarchy and and use VL to control the length of your box via the text's Bounding Box Width Scaled parameter.
2 - Put a perspective camera in the scene and make it active (effectively making your scene live 3D space) and set your White Box's Z position to -1
#XPression