In my very first image on this thread (the room with the wooden floor) the wall intersection has poor geometry (an anomoly with the product that usually doesn't cause issues). I filled that space with a curved primitive (the arch) and coloured it black. Under normal circumstances this was enough to fix light leakage.

My shading rate is always 0.2 or less for finished renders.

The issue, I believe, is that when you bump up the strength of lights and ambient lights it overpowers the geometry. I've had random leaks through very solid interestions at the really high light settings.

Again, this is why I hope SM add a capacity for setting 'exposure' into the cameras. It would reduce blotches, significantly improve render times, and give much greater scope for creative renders.