The same thing happens to me. Exactly the same way Generic explained it.
I apologise for the atrocious sound quality, first time recording and a rather weak CPU and all that, but here, I've recorded the entire sequence from starting the mission to abandoning it due to no update on the quest marker. Of note: I also went the other way around after the Pike despawns (first to the end, then back to the beginning); I've seen the rider despawn in the tunnel right after she summons the first set of mobs. [url]https://youtu.be/N6jQyGWsZ_k[/url]