When you search within the scrollback buffer, the search field is shown in the terminal status line. If the terminal status line is turned off, the search field is invisible.

The search does appear to still work but this may just be confusing if you don't know about it - you hit Alt+Left and nothing visibly changed but your keystrokes aren't going anywhere obvious now.

Ideally in this situation we'd either temporarily turn the status line on for the duration of the search, or perhaps do a Vscrn popup, or a GUI popup window.