How do I stop downward face light discolourartion?

When I try my scene the downward facing face of the top cuboid is an odd brown colour when the material is set to white, similarly all downward facing faces of any blocks also feature the same issue (though for these it is not a problem as they are not visible).

please note that it definitely has nothing to do with the colour of the sphere that contains the light probe nor does it have anything to do with the light probe itself.

Thanks in advance David.