Okay so I ended my flight in ATL From Detroit 6 hours ago and I saw a few pilots flying around. As I am now departing ATL to MIA right now, out of the few I saw earlier, I see one that is still up in the air at the same altitude he was at before I exited the sim LOL. So my question is, is multiplayer really working? I mean unless he’s been circling ATL at the same altitude for the past 6 hours 
Did the community event over London a couple of nights ago and the multiplayer was definitely working then.
https://forums.flightsimulator.com/t/official-community-fly-in-guided-tour-of-london/272571/51
Multiplayer worked for me Friday night and Saturday nigh, on different servers. The guy hanging there might have his sim paused, waiting to get home so he can complete the landing.
What you could try though is switch to another server, go offline then back online again (top right menu when you press esc) or go to settings > data and toggle multiplayer off, save settings, then turn it back on again and save settings.