Don't take my answer as the only solution or top solution, but I believe this is because you are on the free plan and spotify wans to give you an incentive to pay for premium. Sometimes, spotify will give you recomended music instead of just the playlist because they don't want you to be able to just constantly add only the music you want and only listen to that playlist. There might be however a setting for recommending songs that you can turn off which is called Autoplay. But I'm not 100% if that works
