[OnePlus 6T  Questions & Answers] TheGorf:
No problem. It's a OnePlus 6T purchased directly from OnePlus. I don't know what you mean by a "master reset", but I have done many many factory resets. Here's my story so far with my 6T:

I bought it very late in Nov '18 and received it Dec 5th.

The first issue I discovered was that a lot of background apps were getting killed frequently including Waze. It was fantastic on a long road trip when I discovered that it wasn't announcing updates. So I called into support. After a couple exchanges they eventually had me disable battery optimization for everything and that generally settled things down.

That's about when the unreliable text messaging started. I started to realize that text messages were doin one of a couple things:

Sending normal.

Sending but being marked as "undelivered" and deifintely not being sent.

Sending but being marked as "undelivered" but definitely being sent because I would occasionally get replies from the other person.

Support this time recommended a factory reset. Which I did. Within a couple days, it just started all over again.

After a few days of screwing around with it and with support the thing I noticed about this time was that some apps that I thought were installed, weren't. And that I also wasn't seeing any updates. On my pixel 1 I was used to waking up in the morning to find a few apps updated. I wasn't seeing that at all. When I was looking closer I realized that some apps just simply never installed. But after a reboot they would suddenly appear as installed. What I figured out was that updates and installs would download without issue, get to 100% and just sit there. Indefinitely. Like overnight sit there. Eventually it was sorta time out without a fail message. I provided photos and a 10 minute video proving it to support. They really didn't have anything to suggest other than factory resetting again. Everytime I reset it, I could reproduce the issue.

So support finally decided that I needed to send it back to them to "evaluate it" for up to 2 weeks. Ugh. I factory reset it in preparation to send it back (that’s what the documentation explicitly said). However it reset to some weird black screen that just had a white keyboard on it asking for something like "encryption key". But I couldn't enter anything. It was like weirdly frozen like that. Even the power buttons and volume buttons were unresponsive. I called support AGAIN and the woman I spoke with had no idea what it was. Her recommendation was to allow the phone sit as is until the phone ran out of battery. Took about 3 days.

Packaged it up, shipped it in, sat with them for 6 days. The Acer tech "sent text messages and installed apps with no problems" and shipped it back. I unpacked it, started the install and it did the EXACT SAME THING.

I called into support. They suggested factory resetting.

Then I called into support and was put on hold and just ignored. I finally hung up.

I have sent lots of email requests, and the support people has gone completely silent on me.

So I bought a Pixel 3 XL.

So now I have this expensive brick. If I can figure out how to resolve the issues on the phone I can give it to my wife or I won't feel bad selling it to recoup a little money.

And that's what has brought me to this point.