The 2nd mission the "A Naughty Game at the Library" after the briefing from Melia at the library, she told the MC to go upstairs and get undressed. But when I go upstairs, there's no instruction or know how to undress. I try going to each room in the library 2nd floor but nothing happened. But weirdly if the I go to laundromat and speak with the client, I somehow completed the mission. Is this deliberate, or is it bug? could you confirm it? thx in advance, and good luck for your game.

Sounds like a bug, I'll look into it.
You should have access to an event called "Self confidence training" upstairs, in the middle of the bookshelves (there's floating a diamond icon that should be hard to miss)

In my first game I didn't saw any floating diamond. But after starting a new game I could see it.