I wondered myself why searching the playlist acts differently from searching the library or now playing assistant.
I guess it's just supposed to be a quick and dirty cut-down version of the main search.
When searching with the now playing node loaded into the main panel, I'm pretty sure in older versions, you used to be able to press F3 to cycle through the matches. Now it seems to be broken, so that you can only jump to the first occurrence, rendering the search almost useless. There is no 'next' button like when searching the now playing panel (use alt-F to display the searchbox at the top of the panel).
I would suggest posting a bug report raising the issue that there is no way to cycle through the matches when searching the now playing node in the main panel. I wouldn't mind F3 working with the now playing sidebar panel too, rather than having to click the little 'next' button.