Seeking some clarification regarding sorting tracks within a Spotify curated playlist

[user-removed]

So I know the five categories to choose from when wanting to sort within a Spotify playlist are title, recently added, artist, album and custom. With the custom option being the default.

Since I've recently started getting into Spotify curated playlists, I've always left the setting on custom.

So far (for me) every time new songs have been added to a particular playlist, they have been added from the top of the playlist on down.

Because of that, I always knew where the new tracks were going to be added.

I just noticed however that in at least one of the playlists that I follow, that has changed a little. it seems that when new songs were very recently added, they were dropped in somewhere in the middle, or at the very least, not at the very top or bottom of the playlist.

I don't know if that's a glitch or not, but I was just wondering if someone could give me the official way of how Spotify adds tracks to a playlist if the sorting option is left on custom.

Thanks for any help and clarification in advance

Android 6.0.1
Spotify version 6.0.0.864