This is most likely a timing problem. If you type c# fast, then the back button brings you back to the previous query.
If you pause between typing 'c' and '#'. then it brings you back to the previous character only.
That is related to our instant search, where you don't need to hit the return key to search. This causes the problem, that we need to identify when a query ends and the next query starts, to create the correct entries for the back button. We do this via timing - if the pause between key presses is too long, a new, separate entry is assumed. As the DeepHN is a single dynamic HTML page, we need to implement and manage the browser navigation logic ourselves.