Hey, i dont know how you are implementing your level (thats why i made those questions), but i never had that problem, yesterday i ran some simple tests using a hollow block, a model and using sun light and static and dynamic lights, z-buffer and stencil_shadows and the result its always the same: the side of the model affected by the light sources are always brighter than the other side like it should be.
So i think the problem must be in your level or game studio version. If you talk me about the 3dgs shadow system i agree with you, it really sucks! cause z-buffer shadows have hard edges and disappear at some camera angles and stencil shadows kills the framerate.