Perhaps the mixed material is altitude dependant? If that's the case, the cylinder is definitely the highest point of the plane/cylinder combo. I'd suggest editing the material for the plane, using the material editor on the right side of the screen. Just change the name of the material, nothing else. Then, also using the material editor on the right side of the screen, change the name of the material on the cylinder. After that, the material should behave more as expected for either/both objects. Let us know how it turns out! :)