Try loading the lightmap using LoadTexture() and re-applying it to the model. There has always been a problem with alpha+mulltiply with regard to the native .b3d file loader. I found it when I was writing the exporters for LightBulb, but doing what I described used to fix it (for me anyway).
Yep, I figured that was probably the reason it wasn't working in 1.85, but I'll have to update to 1.86 in order to get blended vertex weights going, so I'm going to have to find something that will work with Mark's new way of handling this.