In fact, the vertex buffer reset removal was an overkill from LL's part: I could keep it and simply solved the occlusion culling breakage by not wiping the occlusion data on VB reset, and properly flagging occlusion groups for rebuild.
Fix implemented for Saturday's release. In the mean time, do keep object/object occlusion off while in PBR rendering mode (yes, it sucks, because it impacts frame rates and VRAM consumption)...
If you build the viewer yourself, you can apply this tiny patch to get things fixed: