Magical Water part 2: bugged quest item

when you are taking this quest you are getting divine stone to summon raid boss from the altar at varka. problem is, that when you kill rb and go back to npc to finish quest, quest item remains in your inventory. after making that quest 3-4 times you have 3-4 divine stones there. i didin't remove them by destroying item because i didn't know how it will affect the quest. shouldn't that quest item be removed from inventory when quest is finished?