  Due consideration of proton optical--model potential (OMP) anomalies at
sub-Coulomb energies for medium--weight nuclei is shown to be critical for the
analysis of the unprecedented measurement of $^{59}$Cu$(p,\alpha)^{56}$Ni
reaction cross section at an energy of $\sim$6 MeV [Phys. Rev. C {\bf 104},
L042801 (2021)]. The variation in predicted cross sections from standard
statistical--model calculations and the cross--section range corresponding to
the anomalous proton imaginary--potential depth, for target nuclei off the line
of stability, are distinct and well separated. Consequently, the new
measurement provides, under unique conditions, tests of proton isoscalar and
isovector real--potential components, the anomalous imaginary potential, as
well as previous alpha-particle OMP, for nuclei off the line of stability.
