Is this a bug or some aspect of lighting I don't understand?

Godot Version

Happens in godot 4.2.1(Steam version) as well as 4.3dev5

Question

Hi, I have a SpotLight3D in my character scene and I noticed that this effect happens when I use Forward+ renderer for the game:
Animation

(notice strange shadow that covers left side of the screen)

This effect doesn’t occur if I use Compatibility renderer, which makes me think this could be a bug. But I don’t want to draw any definite conclusions yet since it’s my first 3D game in Godot and I might have just missed something important.

What could this be caused by and if it is a bug, is there a workaround?

The problem seems to be that gltf importer and then the Godot’s skeletal animation system seems to screw up normals of this particular entity. I have no idea how or why this happens, the normals in .blend file and in resulting .glb are fine as far as I can see

Ok I just “solved” this by backing my normal and color maps with Blender, so Godot doesn’t have to calculate normals on its own.
This doesn’t get rid of initial issue, of course. This is just a workaround.