I'm confident even cavemen saw and comprehended shadows against a wall.
Btw, do any of the "humans" in these paintings pass the "not the real thing!" test? It is a painting and in every instance it is clear the correct shadow would mess up the composition.
Next stop an article on "How poets have gotten grammar wrong all these years".
So I am willing to assume that if a shadow fails to climb up a wall, it's because the artist thought it looked better without the shadow on the wall.
But more generally, being among the first to figure things out isn't easy. Nowadays you can look at photographs and paintings like the works of 19th century academic painters and say "this is how realistic paintings are supposed to look", but back in the 15th century all you would be able to see is other people doing it not quite right. I feel like it takes a bit of genius to be the first to do something right, and frankly the average artist isn't that smart.