  In this paper we explore the chiral phase transition in QCD within the
three-flavor Nambu-Jona-Lasinio (NJL) model with a negative coupling constant
in the isoscalar axial-vector channel, which is associated with a polarized
instanton--anti-instanton molecule background. The QCD phase diagram described
in this scenario shows a new first order phase transition around the transition
temperature $T_c$ toward a phase without chiral condensates, but with
nontrivial dynamic chiral chemical potentials for the light quarks,
spontaneously giving rise to local $\mathcal {CP}$ violation and local
chirality imbalance. The corresponding critical temperature $T_{5c}$ for this
phase transition decreases with the magnetic field and it gives a natural
explanation to the inverse magnetic catalysis effect for light quarks when
incorporating a reasonable value of the coupling constant in the isoscalar
axial-vector channel. Furthermore, when the isoscalar axial-vector interaction
is dominant in light quark sector and suppressed in strange quark sector, it is
found that there is no inverse magnetic catalysis for strange quark condensate,
which agrees with lattice results.
