Problem with closed box and material using mix node

[felixrender]

Hi, i did this scene to exasperate the problem. I get the problem only if i use the mix node. For some reason the ground receives some light from the outside, despite the fact that the box is sealed. If i use only one disney node, without the mix node, the problem do not shows up. The file is big only because the size of the textures.

https://drive.google.com/file/d/1-yV6lXEBPgMq5P58LxLsJ9-RRfA1DLMR/view?usp=sharing

Thanks for any help!