  Muon neutrino disappearance measurements at NO$\nu$A suggest that maximal
$\theta_{23}$ is excluded at the 2.6$\sigma$ CL. This is in mild tension with
T2K data which prefer maximal mixing. Considering that NO$\nu$A has a much
longer baseline than T2K, we point out that the apparent departure from maximal
mixing in NO$\nu$A may be a consequence of nonstandard neutrino propagation in
matter.
