Particle rendering bug in 3D scenes

[zxh0916]

Describe the bug
The particles seem to be changing their positions and radius when the camera moving in a 3D scene.

To Reproduce

from math import sin, cos
import numpy as np
import taichi as ti

ti.init(arch=ti.cpu)

faces = np.array([
    [0, 1, 2, 3],  # front
    [4, 5, 6, 7],  # back
    [0, 1, 5, 4],  # bottom
    [2, 3, 7, 6],  # top
    [1, 2, 6, 5],  # right
    [0, 3, 7, 4],  # left
], dtype = np.int32)

indices_array = np.stack([
    faces[:, [0, 1, 2]],
    faces[:, [2, 3, 0]],
], axis=-2).reshape(-1)

# create a box with the given center coordinates and length, width, height
def box_vertices(xyz: np.ndarray, lwh: np.ndarray) -> np.ndarray:
    l, w, h = lwh[..., 0], lwh[..., 1], lwh[..., 2]
    centered_vertices_array = np.stack([
        np.stack([-l/2, -w/2, -h/2], axis=-1),
        np.stack([+l/2, -w/2, -h/2], axis=-1),
        np.stack([+l/2, +w/2, -h/2], axis=-1),
        np.stack([-l/2, +w/2, -h/2], axis=-1),
        np.stack([-l/2, -w/2, +h/2], axis=-1),
        np.stack([+l/2, -w/2, +h/2], axis=-1),
        np.stack([+l/2, +w/2, +h/2], axis=-1),
        np.stack([-l/2, +w/2, +h/2], axis=-1),
    ], axis=-2)
    return centered_vertices_array + np.expand_dims(xyz, -2)

n_boxes = 1

vertices = ti.Vector.field(3, ti.f32, shape=(8*n_boxes, ))
indices  = ti.field(ti.i32, shape=(36*n_boxes, ))
center = ti.Vector.field(3, ti.f32, shape=(1, ))

xyz = np.array([[0.0, 0.0, 0.0]], dtype=np.float32)
lwh = np.array([[1.0, 1.0, 1.0]], dtype=np.float32) # length, width, height along x, y, and z

vertices.from_numpy(box_vertices(xyz, lwh).reshape(-1, 3))
indices.from_numpy(indices_array)
center.from_numpy(np.array([[0.0, 0.0, 0.0]], dtype=np.float32).T)

window = ti.ui.Window("Display Mesh", (640, 640), fps_limit=30)
camera = ti.ui.make_camera()
scene = window.get_scene()
canvas = window.get_canvas()

i = 0

while window.running:
    i = i + 1
    
    cam_pos = np.array([sin(i/60), cos(i/60), 1], dtype=np.float32)
    cam_pos *= (4 + 2*sin(i/30))
    camera.position(*cam_pos)
    camera.lookat(0, 0, 0)
    camera.up(0, 0, 1)
    
    scene.particles(center, radius=0.73, color=(0.8, 0.6, 0.2)) # note that the radius is a constant value
    scene.mesh(vertices, indices, color=(0.8, 0.2, 0.6))
    scene.point_light(pos=(0, 0, 10), color=(1., 1., 1.))
    scene.set_camera(camera)
    canvas.scene(scene)
    window.show()

Log/Screenshots
If you look closely at the overlapping boxes and spheres in the GIF, you'll see that the intersection lines seem to move with the camera's movements.

Additional comments
None