Can you try to run again without the bottom wall BC. MFiX has some default BC wall that I think is the source of the issue. If you have a wall BC defined at the MFiX box boundary, we end up with a “double wall” in the sense that both the default wall and the defined wall BC are triangulated so there are 2 STL triangles colliding with the particle. We should either disable the default wall or provide an option to opt out.