I found this in the manual:
Quote:

Take care that shadow casting lights don't cast a shadow towards the camera. This limitation is normally kept by the sun, but can be violated by low-flying lights.


It's on the cast page and is not mentioned on the shadow or shadow_stencil page. So it's not a bug but a limitation of stencil shadows. I think this should be put on the shadow_stencil page aswell since that's where limitations of stencil shadows are listed.

shader tutorial