I have a problem looting the briar heart aswell, the NPC was a target of 2 quests to begin with, Red Eagle quest and Repairing the Phial quest. Red Eagle quest worked correctly and was able to be completed, looting briar hearts from the quest NPC does not work however. In my save I have killed him twice (waited for respawn) and also looted a briar heart each time but the quest will not update.
Attached a savefile, the NPC in question is probably dead now since it was only days ago I tried looting it in this save if I recall correctly.
__Skyrim Special Edition__ Load Order on PC:
*Unofficial Skyrim Special Edition Patch.esp
*Ordinator - Perks of Skyrim.esp
*Skyrim Flora Overhaul.esp
*RSChildren - Complete.esp
*50 More Perk Points.esp
*Thief skills rebalance for Ordinator.esp
*Apocalypse - Magic of Skyrim.esp
*Apocalypse - Ordinator Compatibility Patch.esp
*Summermyst - Enchantments of Skyrim.esp
*Alternate Start - Live Another Life.esp
*Immersive Citizens - AI Overhaul.esp