The conjecture that higher-rank Clifford indices equal the classical Clifford index

Let $C$ be a smooth projective curve, and for each integer $n\geq3$ let $\gamma_n'$ denote the rank-$n$ Clifford index defined using semistable vector bundles, while $\gamma_1$ is the classical Clifford index of $C$. Equality conjecture for $\gamma_n'$. One has

$$\gamma_n'=\gamma_1.$$

The source calls this the most important point of Mercat's conjecture and notes that it is supported by examples with $\gamma_2'=\gamma_1$, but says that a complete proof seems far off.