In pretty much any KDE app I can think of (e.g. kate, okular), initiating search brings up a bar at the bottom of the window. For consistency, it would be nice if konsole worked like this. One benefit is that options can be access via one click rather than having to access them via a menu, and the width of the search field extends the window (minus the option buttons).
Sorry, this was intended as a wish, not a bug.
If I recall, I think we copied Chrome's browser find dialog. The main idea was not to require a terminal resize (which is very bad IMHO). I'm not positive if the HIG says anything https://develop.kde.org/hig/