Landscape renders default material if apply another material instance under ES31 preview

In ES3.1 preview,

Landscape will render default material if change landscape material by using material instance of landscape's material.

Will render correct if using another material's instance.

1.Create a simple material and it's two instances A & B

2.Create landscape using material instance A

3.Change to ES3.1 Preview

4.Select the landscape and apply material instance B to Landscape Material

5.Landscape now render default material in ES3.1 preview and render material instance B in SM5

6.Create another material and create it's instance C

7.Select the landscape and apply material instance C to Landscape Material

8.Render correct in ES3.1 preview and SM5