We're currently investigating an issue where some mass-start event participants can be routed incorrectly and end up on a different route than is scheduled. This causes them to go a different direction than the rest of the group.
If you experience this issue, submit all your logs from your Documents\Zwift\Logs folder to https://bit.ly/zwiftsupport so we can investigate. If you see someone go off-course, but don't experience it yourself, encourage them to submit logs!
We appreciate your help as we work on fixing this issue.
As there are no workarounds for this issue at this time, our only recommendation is to submit your logs if you encounter this issue. Also, keep in mind that if you log out, you will be kicked from the event and unable to rejoin.