To recap the problem, consider QCD with three massless quark flavors. There is a symmetry $$SU(3)_L \times SU(3)_R \times U(1)_L \times U(1)_R$$ corresponding to independent rotations of the left-chiral and right-chiral quark fields. Vector symmetries are the subset $$SU(3)_V \times U(1)_V$$ which rotate the left-chiral and right-chiral quark fields the same way, while the axial symmetry $U(1)_A$ rotates the fields in opposite directions. Finally, we define $SU(3)_A$ by $$SU(3)_A = SU(3)_L \times SU(3)_R / SU(3)_V$$ and conventionally call it an "axial symmetry group", though it's merely a coset. All of these symmetries except $U(1)_V$ are explicitly broken by the quark masses, but we can treat this as a small effect and ignore it below.
The formation of the chiral condensate spontaneously breaks the symmetry to $SU(3)_V \times U(1)_V$, so we should have $8 + 1$ Goldstone bosons due to $SU(3)_A$ and $U(1)_A$. The $U(1)_A$ problem is the fact that there is no Goldstone boson corresponding to it. The candidate is the $\eta'$, which is much heavier than the $8$ others.
According to most textbooks, the resolution of the $U(1)_A$ problem is that the $U(1)_A$ symmetry is anomalous by a $U(1)_A SU(3)^2$ triangle diagram, and hence not a true symmetry of the quantum field theory. Since it's not a symmetry, it can't be spontaneously broken.
I don't buy this. The problem is that $SU(3)_A$ is also anomalous, by about the same amount. For example, the $U(1)$ subgroup of $SU(3)_A$ corresponding to the pion $\pi^0$ has a $U(1) U(1)_{\text{EM}}^2$ anomaly which accounts for the fast decay $\pi^0 \to \gamma \gamma$. This is important because it's how anomalies were discovered in the first place. So by this reasoning the pion should be heavy as well, but it isn't.
What distinguishes $U(1)_A$ here? Is the anomaly alone really the solution to the $U(1)_A$ problem?