Problem with fix mesh/surface/stress

Hi,

I have build a box with fix wall/gran/hertz/history primitive x-y-zplanes and let the particles grow in there. Now I replaced the top zplane with:

fix zwall_stress all mesh/surface/stress file zwall_top.stl type 1 verbose yes
fix zwalls2 all wall/gran/hertz/history mesh n_meshes 1 meshes zwall_stress

To take a look at geometry and pressure I do a dump mesh/vtk. At the first dumped timestep everything is al right, wall on the right position (z=0.1 from stl file). But on the next after 1000 ts the wall is laying on the 'ground' (z=0). There is no gravity at all, the mesh is not moving. The facets are turned to the inside of the box and get pressure from the bouncing spheres. The zwall_top.stl consists of two facets:

solid
facet normal 0 0 1
outer loop
vertex 0.0 0.0 0.1
vertex 0.1 0.1 0.1
vertex 0.1 0.0 0.1
endloop
endfacet
facet normal 0 0 1
outer loop
vertex 0.1 0.1 0.1
vertex 0.0 0.0 0.1
vertex 0.0 0.1 0.1
endloop
endfacet
endsolid

Created with editor and testet with gmsh.
Replacing the zwall on the z=0 plane with a flipped and translated into z=0 mesh everything is ok, but I like to show there is no gravity and the pressure is independent from direction.
So can anyone explain, why the mesh is moving through the bouncing spheres on to the bottom of the box? Or is that a bug? script and stl are included.

Regards,

Paul