Screencast to demonstrate the issue, more detailed explanation below.
Sorry for the bad quality, it seems that imgur compresses the gif a bit ... a bit much.
Dismissing the Code of Conduct announcement right below the navigation should not affect anything else, but it also dismisses the hero banner. Starting from the situation depicted in the first screenshot, dismissing only the announcement results in both it and the banner disappearing:
I can reproduce this issue on all Stack Exchange communities I have tried it on so far.
Minimizing and dismissing the hero image does not dismiss the announcement though.