I too am having the same issue. I can understand why it happens. There is a slight delay drawing that much foliage even when I play the game in PIE. I’m guessing, although I could be wrong, that when a HRSS is taken, an instance of the level is created and a single frame rendered. My foliage wouldn’t have been drawn by then. Interestingly, it is not the total foliage count that seems to cause this issue, it seems to happen when an individual foliage type reaches this threshold. HRSS work fine for me when I have 40+ foliage types visible and under 1.5k each.
I appreciate this will not be high on the priorities list but with Epic pushing into the real-time VFX area, a delay in displaying the foliage draw call for high fidelity scenes featuring foliage will have to be looked into at some point. For now, I have been able to get around it by creating multiple foliage types of an instanced mesh.
BTW, this is not a complaint, I am loving how much detail I can add to my large open worlds and small micro scenes alike. Thanks Epic 