During the ZTR race today, I noticed that some of the riders who are ahead of me drop out from the rider list, only to reappear later. This appears to be consistent and repeatable.
I recall that the rider list shows riders that are “close” to you - it seems that “close” is a circle centered on the rider, not the road/time distance. The behavior seems that as the rider avatar goes around the the course, if the Euclidean (straight line) distance to the riders ahead of you exceeds a threshold, they drop off from your list.
Pictures and logs available.