Can you explain in detail about the boundary conditions, post some images on your cell zones , mesh and interface? Are you using shell conduction when you say zero thickness wall? Did you merge the zones ?. If you provide these information then I can comment on whether the shadow is in contact with fluid or not .Meanwhile, I recommend you in order to understand the behaviour of the zero thickness with fluid try splitting the cell zones and then get the force plot to compare with the results you already have with shadow wall. I am attaching the user guide link on how to seperate cell zones.