I was using Mobile Link on a Google Pixel running Android 8.1.0. The U-Turn button didn’t work, despite holding it down. I’m not sure if this a recent regression, I haven’t used it before.
By the way, if you don’t minid me nit-picking … I’m really not sure about progress-in-button for mobile. Your finger covers the button, so the only way to see the progress is to try to lift your finger off the button slightly… seems like a poor choice. Works great for mouse-contexts or larger surfaces, I’m sure. But phones? Not for me.
I second this. Had the same problem yesterday. It was hard to see if the process had started (since it takes a few sec) because my finger was covering the entire button. Had to try a few times to turn. I guess the button works, but its just hard to see if you have started the process.
One possible explanation: what was your in-Zwift speed at the time? there is a minimum speed to make a U-Turn.
Interesting, good to know Steve, thanks. I was at a 12% incline and my wheel started slipping (needed to calibrate), so I thought I’d turn around and take the downhill miles for XP, then fix the problem after. But I couldn’t get any speed up, so was just stuck on the side of the hill, nowhere to go! 
Anyway, in this case, I’d like to file a bug report: there is no UI feedback on the Mobile Link for failure to u-turn due to lack of speed, and there should be. When succeeding and failing look the same, you’re going to get annoyed users wondering what the hell’s going on.