The issue is that we don't have acces to the data of the seperate forms.
According to riot, shadow assasin kayne performes alot worse than rhaast. So it's well possible that the shadowasassin is more around ~30%-35% win, which on a rather stright forward kit is actually a sign for needing a buff after 2weeks.
He's not introduceing a completely new playstyle and niche that aligns with broken support itemisation that also was more subtle and need some time to fully get used.
Also, they nerf duskblade, which is core on his assasin form(or nearly any ad champ right now, and while sudkblade and maybe even lethality as a whole is broken atm, it'S again rather clear and straight forward, so not much to learn about itemisation, it's actually more concerning that assasin kayne underperformes despite his core itemisation beeinng potentialy OP.