iPhone deleting songs without permission

For the last 6 months or so, even when I was still working with iOS 10 (iPhone SE), I have had some songs disappearing without any notice.

I don’t have an Apple music subscription.

I sync my music with my iTunes library.

I have always had a lot of free space on my iPhone and I don’t see why it would think it’s okay to delete my songs without my permission.

I can’t find any setting that can disable this feature. It used to be there in earlier versions of iOS 10.

Help me please, thank you