I'm surprised I didn't find any mentions of it. It didn't seem difficult to trigger that glitch! I updated the route several times, and he keeps slipping off as soon as he reaches the endpoint. It's a good thing the visible flight path is there, or I'd never be able to find him afterwards. I'm pretty sure he clipped through the ground a couple of times, too. A really amusing bug.
Funnily enough, the first time I won, I was actually the one to accidentally tumble off as soon as I reached the goal... but it's not like he tracks the player after the race's over, right? Not to mention I changed to a faster route all those times.
There he is, stuck in a tree. Far from the longest fall he took.