Hello BlackForestGames,
There was a change made recently with the release of 4.8 that can explain this behavior. Foliage no longer uses instances clustered into multiple components, and all instances of a particular static mesh are rendered in a single draw call from a single HierarchicalInstancedStaticMeshComponent.
When you paint instances with the foliage tool, the tool finds the Z location matching the surface of the heightfield and adds an appropriate transform to move the instance to that location. In the vertex shader, an ObjectPositionWS material node should evaluate to that same world position for each instance.
Edit: I have been running some test in my own project and was able to confirm what you are reporting. Before I write the bug report for this issue, are there any other notes or points of interest which you can add to the robustness of the report?
Cheers,