So, after downloading your project and experimenting I found a known issue very similar to this. It appears to be a static/stationary lighting issue where a lighting build is required and the engine collects a lighting cache. Essentially what happens is the lighting is derived from a singly source in the cache instead of driving it from the light that is influencing the foliage the most.
Currently this issue is on the TO DO list but I do not have an ETA for when or if it will be handled. The ticket number associated with it is UE-20029.
I have however a work around for you so you can progress with your development. Change all of your lights to movable so that calculation will occur in real time and bypass the cache calculation. Then go to your world settings and under Lightmass click the checkbox for Force No Precomputed Lighting. This will eliminate the build. Lastly, “and please save before you do this as you will need to restart the engine,” go to your Project Settings > Rendering > Turn off the checkbox for allow static lighting.
I was able to to get this. I recorded a movie of some grass that is moving between shadow and light. There isn’t a huge noticeable difference but you can tell that the light is influencing the grass. I apologize in advance for the quality.
