Purchased music randomly deleted

I just downloaded a favorite song again after I noticed it was no longer in my main playlist. It has also been removed from my other playlists. I have played this song about 100 times in the last two months, why would my phone delete it and remove it from my playlists? It's still in my library and easy enough to download, but it's a real nuisance to not know it's been removed from playlists. This has happened to me many times over the years, through many versions of IOS (I'm currently running IOS 11.2.6). I have 74 GB free on my phone, I have NEVER filled memory completely on this iPhone 6 which I've had for 4 years.