Number of people ran into this problem where you notice a tear in the displacement when rendered with mental ray.
If you take a closer look at the render you will notice that this tear appear at the UV border edges, and that is due to the fact that the sub-division approximation is smoothing the UV brooders, that’s why the displacement edges don’t match anymore.
To fix this problem we need to disable this new method of approximation called the CCmesh, by running this MEL command
addAttr -ln “miExportCCMesh” -at bool mentalraySubdivApprox1;
where “mentalraySubdivApprox1” is the name of your approximation node.
However this comes with price, the CCmesh is much aster node to render and once you disable it. You will get a hit at render time.