Categoricity and closure of amalgamation bases under unions

Let $\operatorname{\mathcal{K}}$ be an abstract elementary class, let $\operatorname{Hanf}(\operatorname{\mathcal{K}})$ be its Hanf number, and let $LS(\operatorname{\mathcal{K}})$ be its Löwenheim–Skolem number. For a cardinal $\mu$, let $\operatorname{\mathcal{K}}^{am}_\mu$ be the class of amalgamation bases in $\operatorname{\mathcal{K}}$ of cardinality $\mu$. Closure conjecture for amalgamation bases. If there exists a $\lambda\geq\operatorname{Hanf}(\operatorname{\mathcal{K}})$ such that $\operatorname{\mathcal{K}}$ is categorical in $\lambda$, then $\operatorname{\mathcal{K}}^{am}_\mu$ is closed under unions of length $<\mu^+$ for all $\mu$ with $LS(\operatorname{\mathcal{K}})\leq\mu<\lambda$. This is presented as a weakening of Grossberg's Intermediate Categoricity Conjecture; the source does not give a resolution.