I haven't read the paper myself, only been referred to it. It also violates my expectations about low hanging evolutionary fruit but the link with blindness provides at least some explanation.

Also cannot read your link.

Do you know is that same variant actually in present in the population, or is it just that variants in the same gene haven't shown association? It may be that only that specific variant has the gain of function phenotype, and that it hasn't been present at high enough frequencies in other populations to show up.