I’ve done the Flatland Loop in Makuri Islands a few times now, and it looks like it is supposed to keep a record of how long it takes, similar to The Hilly Loop route on Watopia. There’s a timer running in the corner for most of the route, and it shows data from other riders (4 of them) and my own non-existent records when I finish the route. But the timer disappears when I enter the Castle area (maybe around the 6.5-6.6 mile mark) and doesn’t reappear until I finish the route, when it starts over again at 0.0. This suggests that Zwift thinks I’ve left the route in that area, though I am riding the route that it gives me.
I’m using Zwift in iOS 1.13.2 (71228), running iOS 14.4.2.