Hey MrFondue,
I had the same problem, but it was fixed by going into the materials of the meshes and setting the Dithered LOD Transition option to true.
Hope this helps.
https://answers.unrealengine.com/questions/566089/hierarchical-instanced-static-meshes-no-longer-dis.html
https://answers.unrealengine.com/questions/564333/my-foliage-will-not-cull-since-4150.html