This looks similar to the problem I had with no GI below the equator. While it appeared to be a straight line in mine, different settings did seem to change the shape of the edge a bit and the shape was similar to this.
It got me thinking to possible contributing factors, and as this is over water, and most of my image was water (reflectivity shader only) I tried turning reflectivity off. While the problem didn't go away, the area with GI was bigger, covering part of the top end of Australia. I'm running an uncropped render now (going to take a few hours).
It might be worth seeing what happens with this file, with the cloud layers disabled?
I'll try changing the camera rotation in mine to see if that makes any difference as well.