Viewport setsize errors on simple project with Camera2D and MeshInstance3D

[catafest]

Tested versions

v4.3.dev5.official [89f70e9]

System information

Windows 10 Pro, NVIDIA GeForce GTX 950

Issue description

I tried to use Subviewport with Camera2D and Quadmesh I tested and I followed some issues: #40651
#54122
#50296
#55471
I got these errors :

Maybe this is not a real bug and is a bad practice.
No matter how you use it, there should be no size setting errors...

Steps to reproduce

See the project I added in the section: Minimal reproduction project (MRP) :

Minimal reproduction project (MRP)

SubViewport_Bug_001.zip

[Sauermann]

Possibly a duplicate of #66247

[graphific]

I'm having the same issue on v4.3beta1.
fix for me as mentioned in #66247

func update_material():
	var shader_material = ShaderMaterial.new()
	shader_material.shader = load("res://objects/video.gdshader")
	shader_material.set_shader_parameter("portal_texture", sub_viewport.get_texture())
	mesh_instance.material_override = shader_material

shader

shader_type spatial;
render_mode unshaded;

uniform sampler2D portal_texture;

void fragment() {
    vec4 texture_color = texture(portal_texture, UV);
    ALBEDO = texture_color.rgb;
    ALPHA = texture_color.a;
}