[zowie_vd]

In "Polittico di Sant’Antonio" the cut-off shadow is at the edge of the painting, so I imagine that one might have just been an oversight from the painter. But in general, it is pretty difficult to get shadows right when you're not painting from life since you don't just need to understand how the basic rules work, you also need to be able to imagine how it works and apply it.

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.