According to the Journal of Chemical Education, Volume 80, No.8 (2003); "The first ionization energy of bismuth appears to be anomalous......It has been claimed that spin–
orbit coupling by the Russell–Saunders scheme would lower the ground state of Bi+ ..."
However, the involvement of d and f orbitals in Bi and Po implies that the outermost orbitals are poorly screened hence the drop between nitrogen and oxygen is not observed between Bi and Po.
The same argument could be extended to explain the reason why there not a corresponding drop between Ba and Tl is the sixth period even though they are in the same group as Be and B.
