The Lambert shader does not distinguishing between light sources for creating shadows, thus any light that creates a shadow blocks all lights in that area.
Exactly like the old issue with Phong materials, which has been fixed:
“Shadows should be applied only to shadow casting light.”
Here’s a screenshot of the Lambert and Phong materials and their shadows from two light sources, red and green:
Tested in Chrome only.
Tested on Windows only.
Hardware Requirements (graphics card, VR Device, …)
Running on GTX970.