Have you found a solution (or at least a concrete reason for the issue) yet? I’m also struggling with that (https://answers.unrealengine.com/questions/658509/inconsistent-lod-behavior-on-foliage-and-grass.html)
It’s probably related to the way the engine clusters foliage instances together (it changed from 4.14 to 4.15 and newer somehow). Bug or not, I haven’t found a clear answer to that yet… More related (and unanswered) questions here: