Translucent material on combined mesh blocking GI rays


I have a simple window model, which is just a frame with glass.

The image below shows the window model combined (frame + glass) on the left, and the glass detached from the frame on the right

You can see when the mesh is combined, I am getting opaque shadowing in the gi rays (glass is blocking the light). When separate, it behaves as expected, the light is going through the glass.

Is this a know thing? Is there any way to get this working combined?