Hi! Thanks for the report. I haven't been able to reproduce the issue. Are you playing the web version by any chance? The web build is older than the current downloadable version, so that could explain the difference.
As for the sleepwear, its only purpose is to appear during the ban events.
Nikitami doesn't have a special affection event. Only Celine, Mei, and Yuri do. That said, reaching 50 affection hearts with any character unlocks a unique outfit!