Cannot fill small holes in mesh

[olkham]

I'm having trouble filling small holes in a mesh created from a point cloud. I'm using the following function...

def pointcloud_to_mesh(point_cloud):
    """
    Convert an Open3D point cloud to a mesh using Ball Pivoting Algorithm.
    """   
    distances = point_cloud.compute_nearest_neighbor_distance()
    avg_dist = np.mean(distances)
    radius = 0.5 * avg_dist

    print('Calculating normals...')
    point_cloud.estimate_normals(
        search_param=o3d.geometry.KDTreeSearchParamHybrid(radius=radius, max_nn=30)
    )

    print('Creating mesh...')
    bpa_mesh = o3d.geometry.TriangleMesh.create_from_point_cloud_ball_pivoting(
        point_cloud,
        o3d.utility.DoubleVector([radius, radius*2, radius*4, radius*8, radius*16, radius*32]),
    )


    bpa_mesh = bpa_mesh.remove_degenerate_triangles()
    bpa_mesh = bpa_mesh.remove_duplicated_triangles()
    bpa_mesh = bpa_mesh.remove_non_manifold_edges()
    bpa_mesh = bpa_mesh.remove_unreferenced_vertices()
        
    # Post-process the mesh to clean and fill small holes
    mesh = o3d.t.geometry.TriangleMesh.from_legacy(bpa_mesh)
    filled_mesh = mesh.fill_holes(1000000)
    bpa_mesh = filled_mesh.to_legacy()
    return bpa_mesh 

You can see that some have been filled by the stretched out triangle marked as 'filled' but there are lots of small ones that should be easily filled but are not... what am I missing?

I tried create_from_point_cloud_poisson() from #3913 but the results were destroyed all details.
I'm using open3d 0.18.0 installed from pypi