Quick answer: Godot 4 shader with light_mode skipping a secondary SubViewport? Lights only render in the main viewport - explicitly set lights to render in additional viewports.
Mini-map renders the same scene to a SubViewport. Lights visible in main; dark in mini-map.
Set light render_mode
Light2D's render_mode = Add with additional viewport coverage. Per-light setting.
Or render unshaded
Mini-map uses unshaded materials. Skip the lighting pipeline; consistent visuals.
Use camera-specific layers
Light masks per camera. Main camera sees lit; mini-map sees unlit.
“Lights are camera-scoped. Multi-camera setups need multi-camera lights.”
For mini-maps and inset views, simplified rendering is usually the right approach. Match the main view; you've over-engineered.