There is a current unresolved bug affecting AppleTV users when participating in group rides of > 100 riders. Other rider’s avatars will flicker especially when riding mid or rear pack. Zwift support have been contacted and acknowledged the problem, but cannot give an ETA on resolution. Their recommendation was to use a device other than an AppleTV for group rides until the fix is released.
Search YouTube for “Zwift AppleTV Group Ride Bug” for a recording of the issue.