Looks like you do not have anything on that layer as it is not being recognized in the Extract Assets window.
I tried the same on my end and was able to extract the content of the rectangle with drop shadow applied.
You might be following a different workflow. If you could share your workflow and the exact steps, i will try to replicate the same on my end. Also, share your OS details.
Look forward to your reply.
Look closely at my previous screenshot. There is something on my layer: two black rectangles are visible on my Layers panel and they are also visible on the canvas. So I don't understand how you can say that there is nothing. Why Extract Assets does not recognise what's on this layer: these are only black pixels with a drop shadow.