Forgive my uneducated guess here, but are we sure "shadow" is literal? It would seem that from the
depiction of Ascians in 1.x, they're complete demons that, while rarely seen, are apparently seen enough to strike more fear into people's hearts than Bahamut himself. These Ascians in ARR are more human-like in nature. Perhaps their 'non-shadow' description is more to say that they're not quite
true Ascians? That they're having to overcome a trial or something in order to "evolve" into or something.
In any case, I just don't feel like "shadowless" should be considered literal, or have to do with their actual shadow on the ground. Correct me if I'm wrong, please. I don't have as much knowledge on the matter ^^