Henning–Oellermann–Swart conjecture on the Steiner diameter-to-radius ratio

Let $G$ be a connected graph of order at least $k$, and let $\operatorname{srad}_k(G)$ and $\operatorname{sdiam}_k(G)$ denote its Steiner $k$-radius and Steiner $k$-diameter, respectively. Henning–Oellermann–Swart conjecture.

$$\operatorname{sdiam}_k(G)\leq \frac{2(k+1)}{2k-1}\operatorname{srad}_k(G).$$

Henning, Oellermann, and Swart constructed, for each $k\geq 2$, a graph attaining equality, so the constant would be sharp. The conjecture extends the corresponding bound for trees to all connected graphs; the supplied source does not state whether it has been resolved.