Apple Music keeps removing my downloaded music by itself

I've been using my iPod Touch 6th Gen to play my music in the car using the Apple Music app. I had a few of my playlists downloaded so that I could play them offline. I was having one of my playlists in the car played during one of my trips and my iPod Touch was sort of acting up and it kept freezing and my Music app finally crashed (probably because it's an old device). It's irritating! And when I reopened the app, the downloaded songs in the playlist I was listening to were removed by itself! Another thing I notice is when I try to download another playlist which has over 500 songs, it won't let me download and the download keeps stopping despite the fact I still have a lot of space left in my iPod Touch storage.