As discussed in this topic.:
When I’m riding the Reverse Titans Groove left-right selection stops showing up when reaching the junction at the end, and does not show up before a restarts.
Pressing left-right arrows doesn’t affect the route. The avatar just rides whatever it will.
I’m on Win10 but it seems that the OP of the linked thread is experiencing the same issues on IOS.
It seems that the issue started with the last update, with the MTB track visible.
Hope the Zwift team will look into this.