Apple Music

See step 6 in the 'Create a playlist on your iPhone, iPod touch, or Android device' section of:

How to make a playlist in the Apple Music app - Apple Support

You can also touch and hold a song, album, or playlist. Then tap Add to a Playlist to add the music to an existing one, or tap New Playlist. Or you can touch and hold on a playlist to download, share, or delete it.

So this actually happens to me a lot. The music app glitches (I’m assuming) and won’t reflect that I’ve “loved” a song or that I’ve added it to my playlist. I have to close the app several times in order for it to work. Sometimes after I do that, the playlist shows that I’ve added it like 5+ times lol I wasn’t sure if it’s a bug or just my connection but that’s what I have to do. Hope it works for you!