Differentiability conjecture for holonomic modules over the completed algebra of vector fields

Let $X$ be the underlying smooth variety, and let $\mathcal{M}$ be a holonomic $\widehat{\mathcal{A}\mathcal{V}}$-module. Here holonomicity is the finiteness condition used in the paper for these modules. Differentiability conjecture. Every holonomic $\widehat{\mathcal{A}\mathcal{V}}$-module is differentiable. Differentiability is important because it permits the local algebraic description used later to prove finite length and classify composition factors. The source states this as a conjecture and gives no evidence of resolution.