If an object has selection sets, the material applied "underneath" (ie. furthest to the left of the stacking hierarchy) will override and effect all the other materials DISPLACEMENT channels unless this material tag has a selection set itself.
If this material was stacked furthest to the right I would expect something like this result as it would become part of the new material stacking workflow.
If the object has numerous materials stacked, sometimes I keep multiple materials on an object for different options and I don't want to delete them, all the channels except displacement are driven by the material tag furthest to the right.
So if the material furthest right has displacement but the one furthest to the left does not, no displacement will be rendered. Mix textures IS NOT enabled either. Perhaps this behaviour would be expected if Mix Textures were enabled.
Either way, all the channels are driven by the material stacked furthest to the right except displacement which seems incorrect to me.
