Thanks. You've explained and showed what you see in Spine, but what do you see when you export? What export settings do you use?
BTW, make sure you are really using 4.0.56. A few versions older had an issue with additive and image/video export.
Using 4.0.56 I made spineboy's head additive and exported with a background color. I don't get black behind the head.
If you are exporting for images or video, why are you using additive? You are likely better off using normal blending.