Hi,
This happens because your foliage material(s) do not have the “Dithered LOD Transition” checkbox checked. Without it in 4.15 instances will not fade out. Previously if this checkbox wasn’t checked, dithered LOD transitions would occur but would render an ugly black artifact as well.
The flag is under the “Material” category in the advanced properties rollout.
Note that this flag must be set on all materials used on all LODs of your foliage mesh for the dithering and fading to work correctly.
I opened the content you linked above and confirmed that fixes the issue.
Cheers
Jack