This will need an example to be made clear. Choose an artist in your library that has two words in its name. I will use the band 'White Stripes' for my demonstration, but 'Bon Jovi' or 'Van Morrison' would work equally well.

1) Select your entire music library on the left-hand pane of the GUI, and then give focus to the search box.
2) Type the string 'White' and wait until the GUI is updated with all songs containing the search term. 
3) Now type a space followed by the word 'Stripes'. The GUI is updated to restrict your search results to all songs that match the string 'White Stripes'.
4) Clear the search box, and give it focus again. The GUI should reset to show all songs in your library
5) This time, type the word 'White ' in the search box, and be sure to include the trailing space character. The GUI is updated to show all songs that match the string 'White' (the space is ignored), but more importantly, the carat is reset to the start of the search box.
6) Type the word 'Stripes' to continue narrowing your search. Because the carat has been moved to the start of the search box, your search string is now 'StripesWhite' and when the GUI updates, you won't get any results.

This is a real annoyance because if I hesitate while entering a search term, the carat is moved and my search is ruined. It should be a one-line fix - somehow, that carat is being moved. Perhaps focus is being taken away from and then given back to the search box?