I don't know what causes this, but sometime when I add a keyword to a issue and
hit "Apply", nothing happens.

Can anyone reproduce this?

As far as I am concerned I just type the keyword I want directly.

I'm only experiencing this when I try to add it to somebody else's issue. For 
instance, right now while viewing this Issue, the "Apply" button won't work; 
however if you got to create a new Issue, the Apply button will work. I think 
it may just have to do with who owns what.

Is it occurring while creating a new one, or only at all other times?

"...however if you [go to] create a new Issue..."