Downloaded playlist of local files grays out every few weeks

[OdayDolphin]

Sorry if this is a repost, it failed the first time I tried to post this.

I have a premium account, I listen on a Samsung Galaxy S8 and my Windows PC.

I have a playlist of music that I own, they are local files on my PC.  This playlist is downloaded to my S8.  It usually works fine, I am able to "Like" the songs on it and add them to other playlists, as well as listen.

BUT, every 2-3 weeks, the whole playlist grays out and will not play on my phone.  It still works fine on my PC.  After a lot of troubleshooting, I figured out that it will be fixed if I completely delete and then re-download the playlist on my phone.  This is irritating, as it is the only offline playlist that I have and I use it when I have no internet connection.

Why does a playlist of my non-spotify owned music stop working on my phone and need to be re-downloaded every few weeks?