@andrewilley correct me if i'm wrong, I've just tested it to be sure. But if you search or go into album view and then go into that album to select tracks you have to add them into the playlist right away before exiting the album or performing another search. Every couple of tracks you need to hit the kebab menu and add to playlist (Also need to select which playlist). And if you add the wrong song you need to go to the playlist and remove it.
it works on Blackplayer that once you are in the playlists "search" mode you type in what you want to find, tap the track and in it goes, no need to navigate to a menu and select "add to playlist". Press the wrong track and the highlight over it disappears (no longer in playlist).
It makes playlist creation and management much more purposeful and a lot more enjoyable to build.
Got burnt by BP deleting my playlists one to many times and don't want to risk building them up all over again, but seems to be the only app that gives you both options.