Adding and removing songs from playlist in Spotify is unclear

Hello,
I started using Spotify yesterday after a significant pause, like a year or so and noticed that adding and removing songs from playlist is not clear for me.
When I press "Add to playlist" button and double tap on a needed playlist it is stated as selected, however the song is not added to it.
The same thing happens when removing the song from a playlist.
I double tap on a playlist where the song is present, the selected state is removed but if I press close button and look again the song is still there.
How do I add/remove songs in playlist using "Add to playlist" button?
Thank you!