The results from using 8 are simply wrong. It converges, but not to any sensible physical solution. If you look at the values of the transmission spectrum, they are all zero (or 1e-22, which is zero).
In my own experience, changing this keyword should only be necessary in extremely difficult convergence cases. If your calculation with 0 converges, there is no reason to change it, you will not converge faster by modifying it.
There is probably another reason why the finite bias calculation doesn't converge. Perhaps you have a perfect ribbon without scattering? That is difficult - and pointless - to treat at finite bias.