If she were facing forward that would have clearly hit Kayle.
It's just an unfortunate side effect of perspective - the only real solution would be to remodel/animate Kayle to fill all her hitbox more often (maybe larger wings, or to move in poses where she rakes up more room, etc). Aether Wing Kayle actually does a fairly good job because she flies slanted but her wings don't move as much.
And you can't just "reduce the hitbox" - Kayle's hitbox is circular, and it is probably already smaller than the full extent of her wings (similar to Monarch Kog'Maw and Kayle) - but it has to include part of the wings or you'll get situations where it visually hits the wings while she's facing forward but it doesn't "hit".
P.S. LeBlanc's E probably is generous at the moment - but this image is largely Kayle's fault.