Music syncing troubles

I have about 25,000 songs in the Music Library on my iMac. (None of them are from the Apple store, these are are songs and books I've imported directly into the iMac.)

When I sync my phone to the Mac (I have 1 TB of memory on the phone) the process keeps downloading about 3,500 songs -- even though those songs are already on the phone. It doesn't do it with all 25,000, just these 3,500. Some of them will be a particular song from an album, but not all the songs from that album are being "re-synced." I've tried deleting those songs from my iMac library and re-importing them. but it's not making any difference.

I've talked with Apple Care and they were clueless and had no suggestions as to why it's happening or how to fix it.

Anyone else have this problem?