I've changed it to using a similar background color and the problem persists.
Hmm, it changes with what I have selected as well. It I have the object selected it doesn't have the middle bit, if I don't have it selected, then it does have the middle bit. Weird. Are you optimizing the opacity when an object isn't selected?
ON a more general note, I'm not sure that additive alpha is correct. If I have an additive-alpha material with the colour:
Now if I have two objects, one behind the other, then the colors should add and become:
But both the overlapping and non-overlapping bits are both the original material color. Overlaying two additive alpha objects doesn't add the colors.
Or am I misunderstanding Additive Alpha?