Regression checked in //UE4/Release-4.26.2 CL 15973114 the issue is not reproduced thee
The problem is reproduced when you open the attached project NewWorld.umap.
The steps to reproduce it from scratch are as follows
1. Create a Landscape.
2. Create two different LandscapeGrassTypes, create a LandscapeMaterial for each, and set each to a LandscapeComponent.
3. The overridden LandscapeMaterial's Grass is not displayed.
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-126118 in the post.