This is already known.
It is not a bug, it has to do with the way Jhin's attack speed works. Jhin's base attack speed is semi-static. It increases with level, but cannot otherwise by changed. Positive changes to attack speed are instead converted into an AD modifier. Negative changes to attack speed attempt to reduce his base attack speed, but cannot.
Currently, Frozen Heart and other attack speed reductions attempt to lower attack speed, so if they worked at all they would only affect Jhin's attack speed. As all of Jhin's bonus attack speed never actually affects his base attack speed, it doesn't really make sense for Frozen Heart, etc., to lower his attack damage. It would work that way if Jhin's passive functioned like Hecarim/Rammus/Galio, where one stat is kept, but also converted into another stat. It does not work when Jhin's passive makes his base attack speed semi-static.