Songs being deleted every 2-3 days

Hello,

I have had this problem since I updated to iOS 10 and it is really annoying. Some specific songs (about 20-30) keep getting deleted from my iPhone. I have tried everything. I tried syncing from a playlist, I tried adding manually, I tried autofilling from another playlist. Nothing. They always keep getting deleted after a day or tow and I have no idea why.

Please note that all my songs are from my iTunes library and they are not purchased. I don't have Apple Music on and I have plenty of storage on my phone (about 45Gb). I have the latest versions of iTunes and iOS. So I would really appreciate a valid solution for this cause it has been going on since 4 updates ago.