This issue has been reported and discussed often in these forums.
While there is no single cause or solution, your situation may be different from others, so it pays to open a support ticket and have Zwift take a look at your log files and configuration (pc and network).
I have experienced this issue occasionally during my tenure with Zwift since Feb. 2014. I have opened a ticket and we did not come to a resolution. That isn’t the same experience as everyone else so again I would suggest you open a support ticket.
My experience has been that for some reason, I lose Internet connectivity (WAN connection at router) and then the Zwift client has trouble re-connecting to the Zwift servers. Once Zwift is running and you are riding but you then lose connectivity, that’s when riders will disappear. You can continue to ride and complete your ride but in the end when you save your ride, it may still fail to upload, even though you are now connected to the Internet. Your ride files are always saved locally, regardless if you save or discard a ride. That way you can always manually upload then to Strava, etc.