I had the bounty beforehand. Followed the shade into the cave. Killed Belderreno, as someone else observed, only a primal rock dropped.
I reloaded an autosave just after the fight (to see if reloading the area would help), still only a primal rock on Belderreno loot drop.
I reloaded before the cave encounter (before triggering the shade), fought again, this time it dropped.
I noticed it drop when I killed Belderreno (saw the red drop glow), but before I killed the smaller mobs/shard things he spawns on death. Looted it then and there before finishing the fight, don't know if that makes a difference or not...