Kac-type conjecture for the spherical Hall Lie algebra

Let $A_{g,r,d}(0)$ be the constant term of the polynomial counting absolutely indecomposable vector bundles of rank $r$ and degree $d$, and let $\mathfrak{h}^{\mathrm{sph}}$ be the spherical Hall Lie algebra with graded component $\mathfrak{h}^{\mathrm{sph}}_{(r,d)}$. Kac-type conjecture. For every $(r,d)\in(\mathbb{Z}^2)^+$,

$$A_{g,r,d}(0)=\text{dim}\mathfrak{h}^{\mathrm{sph}}_{(r,d)}.$$

This is proposed as the analogue for curves of Kac's conjecture identifying constant terms of Kac polynomials with root multiplicities. The source gives no resolution.