Changing the InstancedFoliageGridSize by the WorldPartitionFoliageBuilder commandlet will cause some of the foliage to not load correctly.
1. Create a new project and create an OpenWorld map.
2. Paint near the origin using the foliage tool.
3. Close the Editor.
4. Execute the WorldPartitionFoliageBuilder commandlet and set "-NewGridSize=51200".
5. Open the Editor
6. Open the map from Step1, and PIE.
Then it appears that only about 1/4 of the foliage area is loaded.
1. Download the project from the comments link.
2. Open the map "LV_Sample1_Source".
3. Run PIE. The foliage around the cube placed at the origin. (Sample1-Base(OK).png)
4. Exit PIE and open the map "LV_Sample1_Cnverted" converted with NewGridSize=51200.
5. At this time, The foliage around the cube placed at the origin is partially missing. (Sample1-Converted(NG).png)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-155064 in the post.