I'm in a similar situation with Apple Care+ Express Replacment: iPhone 6S+ 64GB Space Grey.
I have a bad Ear Receiver. The Ear Receiver makes a rattle sound when I talk to people.
I talked to Apple yesterday. Was on hold for 35 mins to find out that it's completely backordered. I called Apple Care, asked for repair status, and it connected me to a someone in the Repair Status area. She proceeded to put me on hold and had to central dispatch to find that out why my order hasn't shipped. She was informed that have no ETA on when they are receiving replacement stock to fill these. She said wait till Friday and call back again.
EDIT: I also made an Genius Bar appointment after that phone call for later this weekend. See if I can potentially get some resolution there.