Not sure if anyone else is able to replicate it since it only happened on an off chance.
But after you kill Kulle. if you dont grab the soul stone, if the one of the group members leaves and rejoins they keep getting the quest experience!
Dont know if their needs to be a specific number of players but we had a party of 4 and one of the guys left and came back and got the quest experience a load of times before we all got bored.
Im gonna try replicate this later on with Kulle and Azmodan and see if it works. Can someone else confirm?