You can animate a silhouette, it’s just another abstraction of form. If you can animate a stick figure, a distorted long late-afternoon shadow, or amorphous shapes like the pink elephant sequence in Dumbo, you can animate a silhouette. There’s some stylized clean solid black silhouettes in the opening of Batman the animated series.
Okay let me try to restate it, with Disney animation most characters (including Beast) have one animator that does that character. When they animate Disney cells they are told to follow the contours and movements of live-action reference shots they’re given, not to invent the animation themselves. Many shots even got reused in several Disney animations for example the ballroom dance is a shot reused from Sleeping Beauty. There’s nothing in that process that allows an animator to ever animate a silhouette as a black shadow. Also the animators were not the ones in creative control, it’s a lot easier to get your animator to draw the figure first and then alter its appearance as necessary for the final film. So what the animator for Beast intended and what the director intended for those early Beast scenes could be different.
Just commenting that I’m surprised that the original intended version (that I agree should be distributed as is) looks strange to me. It’s a weird fall-off, a separate aspect from how dark the film is. Something can be dark and not have a such a steep notch in a very specific area of the contrast curve. I don’t doubt that it’s intentional.
If you like the film I suggest arranging to see it on film some time if you can.