Here's a cap displaying the unwanted results: Construct Classic r1.2 cap
Here's two examples in a more complex implementation. On the left you'll see the actual render and on the right a sketch of how the shadow was expected to be rendered.
Using the bounding box as the caster shape does not come into question with the said implementation as we'd have to create tricky workarounds involving invisible collision boxes to prevent ugly artifacts due to the nature of the sprites. That way we'd also lose, duh, the custom shape of the shadow we wanted to begin with.
Sorry I didn't make a report to the tracker, Sourceforge is giving me a http error 500. :(
I'd also like to point out that this problem that prevents setting Light-object's height and width in the runtime has yet to be solved and it's hindering our development of the lighting code.
Edit: fixed the image urls and linkified the cap url
Edit 2: A solution presented below