What is the cause of these pixel artifacts and how do I prevent it?

I'm getting these strange image artifacts when I attempt a low-res 3D orthographic scene in Godot 4.4:

Here's my steps to reproduce:

1. In MagicaVoxel, export the built-in "castle" example to *.obj format.

2. In Godot 4, start a new project and set up a basic 3D scene (ground mesh, camera, directional light).

3. Drag-and-drop the castle obj file into the scene, and create a basic BoxMesh3D as well for comparison.

4. I'm going for a faux-2D orthographic look, so I follow the initial setup recommended in this video:

   • Set the project Viewport to 1080p.
   • Put my basic 3D scene under a SubViewport, and put this SubViewport inside a SubViewportContainer
   • Configure the SubViewportContainer to stretch to fit the canvas, and raise "Stretch Shrink" to 2 or greater to approximate a pixelated look.
   • Turn on "Nearest" texture filtering on the SubViewportContainer.

5. Already the preview rendering shows the artifacts noted above; running the scene continues to show the rendering artifacts.

6. Some assorted hypotheses I've had and things I've tried:

   • The Display > Window > "Stretch mode" project setting does not appear to affect this problem one way or the other (at least the disabled and viewport states).
   • The banding seen on the castle also affects the native Godot mesh in certain viewport resolutions--not seen in the screenshow above, but trust me :).

7. As a last try, I tried switching the renderer from "Forward+" to "Compatibility" mode. Alas, that seemed to fix the issue, albeit with a new lighting issue:

So I'm left with a lot of questions:

• Why is my debug run still appearing to run at 1080p or a resolution higher than the shrunken 540p or 360p? Is this an anti-aliasing setting that I need to disable somewhere? Is anti-aliasing the culprit?
• Is this a known issue with the Forward+ renderer?
• Is there some other configurations I could try (e.g. choose a driver) to try to mitigate this issue? I'd like to use the Forward+ renderer if possible.

Thanks in advance for any help or advice at all!