Hot corner doesn't work in overview when there is a fullscreen app "covering" the top bar outside the overview
If an app is fullscreen, and either in the foreground, or not covered by another maximised window (i.e., when the top bar is not displayed as a result of the fullscreen app), then the top left hot corner doesn't work even when in the overview (to exit the overview).
I'm not sure if the hot corner as a means of showing the overview is deliberately disabled when the top bar is hidden, or if it's just an unintended artefact fullscreen handling, but either way, it should still work when in the overview.