try to apply a threshold on geometry first in order to remove the material number 0 (ghost-) layer and the vorticity artifact.
At the top of your channel the highest finite velocity in your simulation at the slip boundary cell has a neighboring cell in the ghost layer with zero velocity. This is part of how OpenLB realizes parallelization and some interfaces and nothing wrong with your setup. ParaView calculates the vorticity based on that huge gradient at the top now resulting in what we can see on your pictures. If you have a look at the geometry field in ParaView you will always see the material 0 layer wrapping the entire domain.