Nonplussed is also used to mean "uninspired" or "unimpressed" - like when something is perhaps a little out of the ordinary [there is an expectation that it will have an impact one way or another] but still doesn't really have an effect on; almost exclusively in the phrase "I was pretty nonplussed by it all" or a close analogue of that.
Certainly this is the usage I've heard and used for the last 20 years or so.
Certainly this is the usage I've heard and used for the last 20 years or so.