Hello! When you invite Koan to walk around, he'll be in your party and that's why he disappears from his table (like story-wise he's walking tgt with kuon, albeit not shown). you can make him leave party/appear again by interacting with the sparkle near his table :D
when suggesting nao and kesa to walk, they'll leave their tables and go outside to sit on balcony. You can check outside (the right door on 2nd floor)