How foliage density impact foliage lighting?

I currently have a problem with the rendering of my foliage when I decrease the density foliage of the engine sclability system.
in epic mode:

in high:

Would anyone know where it came from and how to fix it?

I’m now pretty sure it’s a lightmaps problem. I temporarily fixed it by changing the Lightmaps Type to “Volumetric” in my Foliage Type, however this can increase the performance cost.

I hope someone will find a better solution!