When a material instance is applied to a landscape in a scene and the user resets param in that material instance, that edit is not applied to rendered landscape (while it is if the material instance is applied to a static mesh).
I attached athe project and a screenshot of the problem. If you load the project's Untitled map, you see a white box and a white landscape.
Set Param in NewMaterial_Inst to 0, manually. Box and Landscape are now black.
Reset Param to its default , by presseing on the reset arrow next the the value of Param.
Cube is now white again, while landscape is still black.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-198919 in the post.