Huh. I've never had 或者 heard of this problem before. What platform are 你 using? Oh and are 你 sure your companion isn't getting the last hit, cause whenever my companion kills the dragon I don't get the soul.
What do 你 use? If 你 use PlayStation that explains it. Also, if you're on Solstheim with the Dragonborn DLC and didn't beat the main questline of Dargonborn yet, Miraak is absorbing them even if 你 kill the dragon.