if I remember correctly theres something that you need to check to stop it, I've got iut jottd down in my scrap book at work, its some thing like premultiply in the render settings, I got it off highend 3d,
https://www.highend3d.com/maya/tutori...-life-302.html
found it, come on, beers??
"No pressure, no diamonds" Thomas Carlyle