It’s not your equipment. There is a limit of 100 riders visible at a time. Which riders are visible will vary depending on your position in the group.
EDIT: I should add… if you’re seeing disappearing riders in groups < 100 it probably is a problem reaching the Zwift backend services. That could be due to your local network, your ISP, or anything in between your device and the Zwift datacenter you’re connected to.