This is due to how outline is done. TO draw an outline, text is drawn multiple times. First in black, then in your proper color (in your case -- pink). When you fade a label, it does the same thing, just with a semi-transparent alpha. In your alpha = 1.0 case, you only see black on the edges. Pink colors the center fully. However when you fade something, the blackness can be seen through it.
There is no way around it other than creating this font effect in photoshop instead of using the inspector-chosen effect.