Maybe I'm mistaken then. I assumed that it was the same code since the interface look is identical, but apparently that is not the case. Maybe it actually always worked in playlists. It is in the huge Liked Songs list where I really need the feature and sorely miss it.
Edit2: Knowing that it works in playlists gave me a workaround. Creating a playlist with all my Liked songs and filtering in that playlist. Instructions:
Create a new Empty playlist
Click Liked Songs in the sidebar
Click the first song
Scroll all the way to the end of liked songs
Hold down shift and click the very last song to select all songs
Drag and drop the selected songs onto your "Liked Songs" Playlist
Spotify now goes partially unresponsive for a while if you have a large set of liked songs and/or if you have a slow computer. Be patient.
Profit: You can now filter all your liked songs sanely in the new playlist
To keep the list up 2 date delete the playlist and start over. That will be far easier than searching through it for songs that you have unliked.
Ctrl + A does not work. Has a bug. Does not select all songs.
Right click menu to add to playlist does not work for me. Nothing happens.
I'm on 18.104.22.1684.g136419d9 and now the filter playlist feature works properly with both spaces in the search string and the ability to search across multiple columns at the same time.
But why in the world don't you publish release notes with your updates so that users don't have to accidentally discover that they can now use things properly?