iTunes deletes all iPhone music

I have my music stored as lossless .m4a files on a network share. The iTunes library itself is stored locally. If for some reason that network share is temporarily unavailable (usually because my Mac has timed out and disconnected the share), and I sync my iPhone with iTunes while the share is unavailable, iTunes will delete every song on my iPhone without prompting. When I connect to the share and sync again, it will sync each track individually. For me this is almost 5,000 songs and takes several hours each time it happens. How do I prevent this? Local music storage is not an option.