  The QCD axion remains experimentally viable in the mass range of O(10 MeV) if
(i) it couples predominantly to the first generation of SM fermions; (ii) it
decays to $e^+ e^-$ with a short lifetime $\tau_a\lesssim 10^{-13}\,$s; and
(iii) it has suppressed isovector couplings, i.e., if it is piophobic.
Remarkably, these are precisely the properties required to explain recently
observed anomalies in nuclear de-excitations, to wit: the $e^+e^-$ emission
spectra of isoscalar magnetic transitions of $^{8\!}$Be and $^{4\!}$He nuclei
showed a "bump-like" feature peaked at $m_{e^+e^-}\sim 17$ MeV. In this
article, we argue that on-shell emission of the QCD axion (with the
aforementioned properties) provides an extremely well-motivated, compatible
explanation for the observed excesses in these nuclear de-excitations. The
absence of anomalous features in other measured transitions is also naturally
explained: piophobic axion emission is strongly suppressed in isovector
magnetic transitions, and forbidden in electric transitions. This QCD axion
hypothesis is further corroborated by an independent observation: a
$2-3\,\sigma$ deviation in the measurement of $\Gamma(\pi^0\to e^+e^-)$ from
the Standard Model theoretical expectation. This article also includes detailed
estimations of various axionic signatures in rare light meson decays, which
take into account contributions from low-lying QCD resonance exchange, and, in
the case of rare Kaon decays, the possible effective implementations of $\Delta
S=1$ octet enhancement in chiral perturbation theory. These inherent
uncertainties of the effective description of the strong interactions at low
energies result in large variations in the predictions for hadronic signals of
the QCD axion; in spite of this, the estimated ranges for rare meson decay
rates obtained here can be probed in the near future in $\eta/\eta^\prime$ and
Kaon factories.
