  Carbon atom vacancies in graphene give rise to a local magnetic moment of
$\sigma+\pi$ origin, whose magnitude is yet uncertain and debated. Partial
quenching of $\pi$ magnetism has been ubiquitously reported in periodic
$first-principles$ calculations, with magnetic moments scattered in the range
1.0 - 2.0 $\mu_{B}$, slowly converging to the lower or the upper end, depending
on how the diluted limit is approached. By contrast, (ensemble) density
functional theory calculations on cluster models neatly converge to the value
of $2$ $\mu_{B}$ when increasing the system size. This stunning discrepancy has
sparked an ongoing debate about the role of defect-defect interactions and
self-doping, and about the importance of the self-interaction-error in the
density-functional-theory description of the vacancy-induced states.
  Here, we settle this puzzle by showing that the problem has a fundamental,
mono-electronic origin which is related to the special (periodic) arrangement
of defects that results when using the slab-supercell approach. Specifically,
we report the existence of resonant states that are $anomalously$ delocalized
over the lattice and that make the $\pi$ midgap band $unphysically$ dispersive,
hence prone to self-doping and quenching of the $\pi$ magnetism. Hybrid
functionals fix the problem by widening the gap between the spin-resolved $\pi$
midgap bands, without reducing their artificial widths. As a consequence, while
reconciling the magnetic moment with expectations, they predict a
spin-splitting which is one order of magnitude larger than found in
experiments.
