I'm not able to reproduce that issue.
However I noticed that if I start typing in the search and matching users are already loaded in the store, those results get automatically highlighted and move the focus away from the "search all discussions" link in the dropdown.
Maybe something similar happens with the discussion search, where one result gets highlighted and becomes the result selected by the Enter key? Do you see the result highlighted before experiencing the issue?
Additionally it seems like everytime you click on the search field to regain focus, the last entry is marked as active. This might be a side-effect of the mouse over detection.