I’m experiencing similar behaviour.
When I’m in a online session players keep disappearing and popping up again eventually.
This hppens the whole session.
I hope too that this gets resolved in time.
I got my hopes when they release the Reno air races as these require much more precise positioning for each players aircraft.
https://msfsaddons.com/2021/08/24/competitive-multiplayer-coming-to-msfs-with-the-reno-air-races/