Why does apple keep removing my offline downloads?

Every time I download a playlist for offline listening, it will randomly remove the downloads and I don’t realize this until I’m driving because the songs will lag as in it’s trying to stream it off my data. So no service = no streaming. It’s happened 3 times already where I have to wait to get home on wifi to re download the playlists. Why does it keep undoing the download?? I’m honestly about to switch to spotify.