Open attached sample projects in ImpostorTest427.zip and ImpostorTest500.zip
Open LV_ImpostorTest.umap and compare results on viewport
Result in 4.27 : Instanced Impostors are rendered as expected.
Result in 5.0 : Instanced Impostors are rendered in black.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-156513 in the post.