Fullscreening an application disables the hot corner--from what I can tell this is true of all applications.  I imagine this was probably implemented on purpose: probably someone felt that disabling the hot corner might be nice when, for example, watching a movie in fullscreen.  I don't agree with this, but I can imagine it.

However, there are some applications for which it really doesn't make sense to disable the hot corner when fullscreened.  For example, I've been using gnome-terminal in fullscreen mode lately.  In all other ways it behaves nicely, but if I want to switch viewports or applications I have to use alternatives to the overview because the hot corner doesn't work.

In my opinion the best option is to keep the hotcorner enabled for all applications.  But if there's a legit reason for some applications to disable the hotcorner when fullscreened, at least make it possible for applications, like gnome-terminal to not have this trait.