I created an account just to see if my information could help OP. I was and and still kind of in the same situation. My HTC U11 had the touchscreen bug that would cause lag and random actions on the phone. So I contacted HTC about a repair and they said that it would take 20-25 business days because of a delay of parts. I was like ok that's fine, I'll deal with it.
I created the ticket 10/10/2017 and sent the phone that same day. The phone arrived at the repair center on 10/16/2017. Weeks went by and I heard no updates on my phone. I would call and get nowhere. The support system has poor communication. The repair center can doesn't seem to update the ticket accurately and the support staff pretty much doesn't know anything besides what the consumer does.
So by the time my repair ticket status was updated, it said they received the phone 11/08/2017. Naturally this was not accurate as I sent it via USPS with signature and tracking so I know this was not accurate. So I called and called and got nowhere. I got frustrated and started looking around and found out some users with the same issues would contact Andre Lonne, the president of HTC America, via LinkedIn. I sent him a message explaining my situation and received a response from him saying he would forward the information to Lisa Ordell, the customer service manager.
I received an email from Lisa saying that she would look to get this resolved. I did not hear back from her for a little less than a week. I then contacted Andre again and told him I did not hear back from her and that he would follow up with her. So this is where things start to move forward a bit.
After a few days I called the customer support to check for an update and to just let them know how frustrating of an experience this has been. After getting off the phone with them, I received a call back from the RMA department at HTC offering a promo code for a brand new HTC U11 to make up for the problem that has been this repair process. I gladly accepted but of course the promo code did not work. I called back and they said that it was strange that it did not work and that they would get back to me in 24-48 hours; of course this turned into close to two weeks and still nothing.
So I just kept being persistent and letting them know this is not good customer service. At this point I was looking at possible legal action. An interesting note is I asked a representative how long HTC can legally hold a phone for repair and they replied with until the warranty ends which I thought was strange. Anyways over this week, I returned home from my girlfriend's place and I see this package in front of my door. I opened it to see a new HTC U11 sent from HTC. I was shocked because I was not made aware that I would be sent this. The tracking on the package says it was dropped off 12/20/2017. So it sat there for a day without my knowledge and luckily nobody came and took it. I'm just glad this whole ordeal was over with I will probably call this week though confirming that I received the package and to close the ticket.
My advice to OP is to reach out to Andre Lonne and go from there. Calling the corporate number hangs up on you so that doesn't help. Be persistent in contacting anyone at HTC and letting them know your situation and how unsatisfied you are with how it has been handled. If it gets to a point where nothing is working, look into filing a small claim.
Keep up updated OP. Hope you are able to get this resolved soon.