It's in the FCOM. One engine above 80%N1, 35% N1 differential between the two engines (CFM56) and some flap down and you'll have the B target. It indicates minimum drag, not zero side-slip.
On the approach, configured for landing (flap full), your power setting will generally be around 70% N1 (aircraft gross weight + 10% is a good rule of thumb) so there will be a side-slip indicator on the PFD, not a B target. If you go around, on selecting TOGA, you'll have a B target.
It's not a simulator anomaly.
Will that do?