For each simple Lie algebra $\mathfrak{g}$ of simply-laced type, Hernandez and Leclerc introduced a certain category $\mathcal{C}_{\mathbb{Z}}$ of finite-dimensional representations of the quantum affine algebra of $\mathfrak{g}$, as well as certain subcategories $\mathcal{C}_{\mathbb{Z}}^{\leq ξ}$ depending on a choice of height function adapted to an orientation of the Dynkin graph of $\mathfrak{g}$. In our previous work we constructed an algebra homomorphism $\widetilde{D}_ξ$ whose domain contains the image of the Grothendieck ring of $\mathcal{C}_{\mathbb{Z}}^{\leq ξ}$ under the truncated $q$-character morphism $\widetildeχ_q$ corresponding to $ξ$. We exhibited a close relationship between the composition of $\widetilde{D}_ξ$ with $\widetildeχ_q$ and the morphism $\overline{D}$ recently introduced by Baumann, Kamnitzer and Knutson in their study of the equivariant homology of Mirković-Vilonen cycles. In this paper, we extend $\widetilde{D}_ξ$ in order to investigate its composition with Frenkel-Reshetikhin's original $q$-character morphism. Our main result consists in proving that the $q$-characters of all standard modules in $\mathcal{C}_{\mathbb{Z}}$ lie in the kernel of $\widetilde{D}_ξ$. This provides a large family of new non-trivial rational identities suggesting possible geometric interpretations.