So the issue you have here is that when you dis-aggregate the measures Tableau is creating a mark for each row. If you look at the bottom right you'll see the number of mark Tableau is plotting (40,272)
Now as they have the same X/Y for each body part they are getting plotted on top of one another, and the tool tip is just for the top one.
If you turn the aggregation back on, and set up the viz as per the below (ensure X&Y are Average and Number of Records is SUM) that should do the trick (notice that the number of marks is now 19)
Hope that helps and makes sense.