  Under the aegis of isospin conservation, the amplitudes in Born
approximation, i.e., considering the only one-photon-exchange mechanism, of the
decay $\psi\to \Lambda \overline \Sigma{}^0+{\rm c.c.}$, where $\psi$ is a
vector charmonium, and of the reaction $e^+e^- \to \Lambda \overline
\Sigma{}^0+{\rm c.c.}$ at the $\psi$ mass, are parametrized by the same
electromagnetic coupling. It follows that, the modulus of such a coupling can
be extracted the data on the two observables: the decay branching fraction and
the annihilation cross section. By considering the first two vector charmonia,
$J/\psi$ and $\psi(2S)$, it is found that, especially in the case of
$\psi(2S)$, there is a substantial discrepancy between the values of the
modulus of the same electromagnetic coupling extracted from the branching ratio
and the cross section. We propose, as a possible explanation for such a
disagreement, the presence in the decay amplitude of an isospin-violating
contribution driven by a mechanism based on physical (on-shell) intermediate
states, that, due to their own nature, should be more effective in the
$\psi(2S)$ decay than in that of the $J/\psi$.
