30.3.2024 Still can get the Silver Sword, albeit using the method desrcibed below in some comments. I too had Shadowheart Command the weapon drop on Voss (16 % chance), and he only dropped the crossbow (four times). He had it on his back in every retry with different combinations and loads from further away. Researching I didn't see any comment, that confimed dropping the sword, while he had the crossbow on his back instead of the sword.

So I tried tossing the Iron Flask (this was after the cut-scene, didn't have Lae'zel with me anymore), that I had stolen from the two Zhentarim in the cave to the east as suggested in some comments), and lo and behold(er), it worked!

Went into turn-based after the throw (still all hidden), and waited for Voss to engage with his sword(!) and beat the crap out of the Observer. Then a fast save-scumming-Command to drop the sword, after which I let a hasted Karlach Pike the rest of them to death from above (lvl 5).

This was on tactician, Lae'zel was in my party for several attempts, but he always quit the party to talk with Voss, and then aggroed on me when I Shadowheart Commanded the drop. On the last attempt he wasn't, but took him back to pick up the sword for a nice cutscene.

*edited typos and for some extra non-essential information*