Classic symptom of a network glitch though loads of people are reporting invisible riders, rider lists disappearing etc so possibly your issue could be one of those bugs being reported. Search for “Missing riders” for any number of bug related threads.
To confirm its not your local network , you can drop a relevant log file onto zwiftalizer.com and check for network errors. This is probably the first place to check. Its rare, but some people also have firewall issues but given you indicated its only last 2 groups rides, unlikely.
Question - outside of group rides, do you see riders (ie if you are free-riding?). If so, I’d be inclined to think you have hit a big. Oh by the way, is it a meetup or normal group ride?