Thanks JCL
Hmm, i cannot walk into shadows anymore with shadow_stencil = 1; without being in trouble. I need shadow_stencil = 2; for that. So that`s unfortunately this workaround out
It has become very visible in a normal scene. Inside a shadow it inverts. Outside a shadow i can see the whole groundmesh as a little shadow. That's why i invested some time here to create a reproduceable result.
Avoiding this artifact would be really great