Closing as won't fix. I'm not sure what we can do about this: look at how shallow the angle is from the shadow casting off the top of the wall. For the shadow shape to really fill in the whole area, it would need to extrude that shadow up to infinity. Sadly computers work with the finite, so it extrudes it a very long way, but then stops. This means it fills the shadow in to a finite area with a right edge (instead of an infinite area), and sometimes that edge is visible on-screen.
The manual entry does say you should avoid moving lights too close to casters to avoid these extreme cases. The algorithm could probably be updated, but I think it would need to be fundamentally rewritten. Realistically I don't think that will happen before C3.