Weapons for the Katana powerset have a weird display bug where they don't appear at certain times, for anyone but the player of the character. While you will see your own katana just fine, all the time unless it's sheathed, just like any other weapon powerset.
However, other players cannot see your sword unless you're animating at the time. When you're in idle stance with your sword out, other players just see your empty hands in katana stance! The sword appears for a split second while you're animating the drawing of the weapon, and it suddenly and instantly appears the moment you start to animate an attack or power, but once you return to idle stance, it is gone.
The same bug existed on Resurgence forever, and they never could fix it, but maybe our lovely GMs here can? ^-^