  We report the latest set of spectropolarimetric observations of the magnetic
$\beta$ Cep star $\xi^1$ CMa. The new observations confirm the long-period
model of Shultz et al.\ (2017), who proposed a rotational period of about 30
years and predicted that in 2018 the star should pass through a magnetic null.
In perfect agreement with this projection, all longitudinal magnetic field
$\langle B_z \rangle$ measurements are close to 0 G. Remarkably, individual
Stokes $V$ profiles all display a crossover signature, which is consistent with
$\langle B_z \rangle \sim 0$ but is {\em not} expected when $v\sin{i} \sim 0$.
The crossover signatures furthermore exhibit pulsationally modulated amplitude
and sign variations. We show that these unexpected phenomena can all be
explained by a `radial crossover' effect related to the star's radial
pulsations, together with an important deviation of the global field topology
from a purely dipolar structure, which we explore via a dipole+quadrupole
configuration as the simplest non-dipolar field.
