voxel_down_sample_and_trace not return the full index of points

**Describe the bug**
I want to get the voxelized point cloud with its corresponding index in original point cloud.
but i found that the total index count does not equals the original point cloud number.
in the case below,it expected to be 40000,but actually i got 6519.which means i cannot get
each voxel's original coordinate.maybe the trace function become useless.

**To Reproduce**
Steps to reproduce the behavior:
voxel_down_sample_and_trace(pcd,voxel_size,min_bound,max_bound)

**Expected behavior**
get the downsampled pointcloud and its corresponding index in original point cloud

**Screenshots**

**Environment (please complete the following information):**
- OS: Manjaro 18.04
- Python version:3.6.9
- Open3D version:0.8.0
- Is this remote workstation?: no
- How did you install Open3D?: build_from_source ( gcc)