I looked at the geometry file in Paraview, and indeed I found a fatal flaw. The no slip boundary extended from the wall all over the outlet. I figured this happened because the cylinder defining the pipe and the cylinder defining the outlet were fit to be touching, but without overlap. This caused the functor assinging the geometry numbers to assining a wall BC to the whole surface of the flow field(including the outlet). When the two cylinders are made to overlap by 2*latticeL, the issue is solved.
Thank you so much for your help!