2D Mobjects being rendered above 3D Mobjects even if they have a lower z-coordinate and z_index.

An example of this issue is with a tetrahedron (Tetrahedron()) and a 2D plane (Square()), where, if you rotate the plane, it would overlay above the tetrahedron even if the tetrahedron has a higher z-coordinate:
`

from manim import *

class Scene(ThreeDScene):
    def construct(self):
        tetra = Tetrahedron()
        plane = Square(side_length=5, color=WHITE, fill_opacity=0.4)
        _3D = ThreeDAxes()

        self.add(_3D)
        self.move_camera(phi=65*DEGREES, theta=-45*DEGREES)
        self.play(FadeIn(plane))
        self.play(FadeIn(tetra.shift([0,0,1])))
        self.play(Rotate(plane, 90*DEGREES, about_point=ORIGIN))
        self.wait()

`
This will output:

https://user-images.githubusercontent.com/90397670/132694874-f888ebd8-7e18-4adf-8c8b-4372aff57f94.mp4

The Square Mobject overlays itself over the Tetrahedron when the rotation animation plays.
z_index() does not affect this outcome, I've attempted to give the Square Mobject a lower z_index multiple times with the same result.
The only way I've found to fix this is by defining the plane as a Polyhedron():
`

from manim import *

class Scene(ThreeDScene):
    def construct(self):
        vertices = [
            [-2.5,2.5,0],
            [2.5,2.5,0],
            [2.5,-2.5,0],
            [-2.5,-2.5,0]
        ]
        plane = Polyhedron(vertices, [[0,1,2,3]])
        for i in range(0,4,1):
            plane.graph[i].scale(0)
        plane.set_color(WHITE)
        tetra = Tetrahedron()
        _3D = ThreeDAxes()
        plane.set_z_index(tetra.get_z_index()-1)

        self.add(_3D)
        self.move_camera(phi=65*DEGREES, theta=-45*DEGREES)
        self.play(FadeIn(tetra.shift([0,0,1])))
        self.play(FadeIn(plane))
        self.play(Rotate(plane, 90*DEGREES, about_point=ORIGIN))
        self.wait()

`
This outputs what I was looking for:

https://user-images.githubusercontent.com/90397670/132695649-6bf88c5d-bb37-4828-9359-ba7d607c8525.mp4

Run the supplied ThreeDScene example with Tetrahedron, Square, ThreeDAxes, camera movement, and plane rotation to reproduce the incorrect overlay. Trace how 2D and 3D mobjects are ordered during rendering and how z-coordinate and z_index are applied; done means the rotated Square renders behind the higher tetrahedron as expected.