This is the result of having the "shadow terminator" fix applied only to the root of the material nodes tree. Your bump map create normals tangent to the floor plane and the rays bounce under the plane (and than hit the sky light source).
However, you can just pick the kind of artifacts you want: bright for receiving light from under the plane, or black splotches for having the "shadow terminator" fix applied.
This explain the different results from plugging the bump map on the mix node ("shadow terminator" fix applied) or on the mixed nodes ("shadow terminator" fix not applied).
At the end of the day, the source of the problem is the bump map used: it produces tangent normals to the floor plane. You can replace it or reduce the scale to alleviate/remove the problem. If you use a smaller scale (aka bump height), you will see the problem fade away (because normals will be less and less tangent to the floor plane).