What I see is Lovelady very close to getting an elbow to the head and until I see proof overwise that's where he was at, within touching distance of that railing.
If this "shadow" is a photographic anomally it wouldn't be the first in this evidence.
Two examples that spring to mind;
later in the Wiegamn film itself, in the best frames available there is no evidence of Zapruder and Sitzman on that pedestal, yet they must be stil there. What have researchers said in the past about that? Evidence of forgery! Another is the Moorman photo.
We know there was someone still stood behind that wall near the corner, he/she is there in Willis and Groden found movement up there in Nix during the shooting ityself and immeadiatly after, and yet in Mary's classic shot we cannot see any sign of this person.
Are these examples exactly the same as the BL problem? No but perhaps still worth considering.