I find it curious that Catwoman can make the string whiff on a mirror Catwoman. I wonder if this only affects the characters you have listed, that those characters have issues with their hitboxes based on their stance.
The characters excluded like Harley, Joker, Batman, probably don't have any problems with one another I would guess. For example, Batman wouldn't whiff against Harley in any circumstance (because their hitboxes are not affected by stance), but he would whiff against Catwoman sometimes because her hitbox changes based on her stance.
I'll test out this theory when I get a chance.