Non-newtonian fluid problem

Hello, everyone
I would like to used the nonNewtonian in Palabos. There is an example in v1.20 which is about Carreau fluid. Now I have a question:for the Carreau fluid, when the exponent n is set to be 1, the carreau model should be reduced to the Newton model. But I can’t get a correct and convergent result for poiseuille flow. I use the same parameters to compare the carreau model with n=1 with the poisuelle flow given in v1.20, and the boundary condition is also same. The first step of results coming from the two models are same, however, from the second step on, the result of carreau model will become more and more disparate? It’s a so odd bug.
I need some help.
Thanks a lot for any possible reason.