Positivity conjecture for exceptional Hall coefficients

Let ${\mathcal C}$ be the cluster category under consideration, let $M$ be any object of ${\mathcal C}$, let $K$ be an exceptional object of ${\mathcal C}$, and let $r(M,K)$ denote the Hall coefficient appearing in the exceptional Hall algebra multiplication.

Positivity conjecture. For any object $M$ and any exceptional object $K$ of ${\mathcal C}$, the integer $r(M,K)$ is nonnegative.

This is presented as an analogue of Lusztig's positivity theorem for the dual canonical basis and would give positivity for the relevant exceptional Hall algebra coefficients. The source provides no resolution of the claim.