Well I'm afraid that's a side effect when "Advanced Queue Mode" is disabled. If this mode is off and you select a song for playback, the queue will always be replaced with the current song and the songs that follow this one. Now, if you search and select a song from the result list, the queue will be replaced with this song and whatever the result list contains after that song.
I'm not sure what another consistend alternative would be in this case. Should only the current song be added to the top of the queue? Or should it be added to the bottom of the queue?
Actually this is exactly what you can do in "Advanced Queue Mode". Here you just add it from the result list to the queue at the top or end depending on your preference.