Weird... it seems that HDRI lighting is causing super brightness in the picture viewer/disk render but not in the viewport render???
Reproduce the issue:
1. Place a camera, light, cube and a sky in the scene.
2. turn on prorender as your renderer
3. render the scene in your viewport using ProRender->StartProRender
4. render the scene to your picture viewport
The picture viewer render is considerably brighter than the viewport render. By enabling a filter I can see it is about 0.5 gamma difference across them.
Why is this happening? It seems like this is not an issue when I use normal lighting/not HDRI sky.