I have yet to do any real play testing of this, and only have seen it once personally. However, the guy I fought said this has happened to him multiple times, so I figured it wouldn't hurt to see if you all had any ideas.
I was dueling a guy in the arena. He was using the Harald Curved Greatsword, while I was using the Frayed Blade because I'm a low life. I tried to hit him with a running attack (because I was using a katana what else would I do), and he timed a swing perfectly, poising through my attack and swinging through me.
And it did no damage at all. It didn't even stagger me.
According to him, this has happened other times, against other katana running r1 (he said specifically frayed blade, but every dex build in the world is using that, so I imagine there is no difference between this and other katanas). I had my camera aimed above us, and it most definitely seemed to go right through my hitbox.
Some possibilities are:
While the Round Plaza is mostly flat, there is a slight height difference in the center. Maybe the mixture of the running r1 animation having my character sort of hunch down, the high swing of the normal r1 of the Harald Curved Greatsword, the small height difference, and the at times stupidly specific hitboxes in this game, all caused the stars to align and the sword passed inches over my head.
Hear about that Stomp thing, where using the WA "Stomp" on the l1 charge of the RKUGS causes you to phase right through the attack? Imagine that but on a katana r1 spam. This is less likely, as there has not been anything mentioned in any of the recent patchnotes to suggest that a significant change has been made to the animations of either weapon. Poise values have been changed in regards to the Curved GreatSwords, but I don't think that would directly apply here. Of course, mistakes happen, maybe they made a small error in the while while modifying the weapon?
Either way this isn't very serious so long as it stays are scarce as it has so far. I've been doing pvp a lot lately, and this is the first time I've seen it. Meanwhile, this guy still kicked the crap out of me. But it is concerning that the weapon can seemingly "phase right through people", even on extremely specific occasions. Hopefully it's just Dark Souls and it's hit boxes striking again, and hopefully it's either resolved, or just completely circumstantial that anyone has even encountered this.