[Geoff Bacon]

The mechanism returns any song that contains the specified phrase i.e in your case, Ballad will return any song that contains that word.
It sounds as though you are wanting to only return those that have both Ballad and Fav.

One thing you could do is put all ballads into a collection called Ballads and all Fav into a collection called Fav (a song can be in multiple collections).

You can then filter by setting the collection to Ballads and the search text to Fav

Not ideal but may be sufficient for your situation.
Note: the filter allows you to specify multiple collections but this returns any  song in either collection whereas you want only those in both selections.

It should be quite easy to add the files into a new collection as you have already stored genre values; set the filter to one of the genre words , long press one of the songs until a check mark exists, use the floating toolbar to select all displayed songs, three vertical dots and "Add selected songs to a collection" (haven't checked the exact wording)

Geoff
PS: You could just name your collections Fav Ballads, Fav Pop etc; having created these collections, you could then remove the genre values (unless you any to keep them)