Spotify adds songs even though I have at least 15 songs in the playlist

[Unvers]

Plan

Free

Country

Italy

Device

Samsung J5

Operating System

Android 9

My Question or Issue

I noticed that Spotify sometimes adds its songs even if my playlist has reached at least 15 songs which is the minimum number to avoid Spotify adding its songs to your playlist, in general, however, if I exceed 20 it stops adding his own and leaves only mines, it is a bug or is it wanted?

I don't intend that it physically add the songs on the playlist where only mine remain but that it add them during playback usually when the ones I added are finished even if the option to play similar songs at the end of the playlist is disabled.