No, it's not a bug. It's just that Blitz3D isn't going to sort them for you. Doesn't take much to sort the polygons based on distance from the camera, just get the distance of'em all (or all those that are facing the camera) inside an array then run a quicksort on it. It's what I do for a great deal of my stuff that uses alpha blending (vertex alpha, typically, but it's generally the same result to me).