Server checks if midround syncing is needed when a client is in the game (i.e. loaded the sub, etc), not when the client requests the start message. Some clients take longer to start the round than others, and they may miss unique messages even if they were in the lobby when the round started.

Clients also get extra 10 seconds to receive newly created events after the midround syncing is done

Subsurface/Source/Networking/GameServer.cs

@@ -565,7 +565,6 @@ namespace Barotrauma.Networking
                         //game already started -> send start message immediately
                         if (gameStarted)
                         {
-                            connectedClient.NeedsMidRoundSync = true;
                             SendStartMessage(roundStartSeed, Submarine.MainSub, GameMain.GameSession.gameMode.Preset, connectedClient);
                         }
                     }
@@ -655,12 +654,8 @@ namespace Barotrauma.Networking
             {
                 if (!c.inGame)
                 {
-                    if (c.NeedsMidRoundSync)
+                    //check if midround syncing is needed due to missed unique events
-                    {
+                    entityEventManager.InitClientMidRoundSync(c);                    
-                        //client joined mid-round and has just started up the game
-                        //check which unique messages they've missed
-                        entityEventManager.InitClientMidRoundSync(c);
-                    }
                     c.inGame = true;
                 }
             }

Subsurface/Source/Networking/NetEntityEvent/ServerEntityEventManager.cs

@@ -133,12 +133,14 @@ namespace Barotrauma.Networking
                 {
                     //it's been 10 seconds since this event was created
                     //kick everyone that hasn't received it yet, this is way too old
-                    List<Client> toKick = inGameClients.FindAll(c => NetIdUtils.IdMoreRecent((UInt16)(lastSentToAll + 1), c.lastRecvEntityEventID));
+                    List<Client> toKick = inGameClients.FindAll(c => 
+                        NetIdUtils.IdMoreRecent((UInt16)(lastSentToAll + 1), c.lastRecvEntityEventID) && 
+                        (Timing.TotalTime - c.MidRoundSyncTimeOut) > 10.0f); //give mid-round joining players extra 10 seconds to receive the events
+
                     if (toKick != null) toKick.ForEach(c => server.DisconnectClient(c, "", "You have been disconnected because of excessive desync"));
                 }
             }
-
+            
-            //TODO: calculate a reasonable timeout period for midround syncing and/or give the client more time if they're receiving messages
             var timedOutClients = clients.FindAll(c => c.inGame && c.NeedsMidRoundSync && Timing.TotalTime > c.MidRoundSyncTimeOut);
             if (timedOutClients != null) timedOutClients.ForEach(c => GameMain.Server.DisconnectClient(c, "", "You have been disconnected because syncing your client with the server took too long."));
             
@@ -248,7 +250,14 @@ namespace Barotrauma.Networking
 
         public void InitClientMidRoundSync(Client client)
         {
-            if (uniqueEvents.Count > 0)
+            //no need for midround syncing if no events have been created,
+            //or if the first created unique event is still in the event list
+            if (uniqueEvents.Count == 0 || (events.Count > 0 && events[0].ID == uniqueEvents[0].ID))
+            {
+                client.UnreceivedEntityEventCount = 0;
+                client.NeedsMidRoundSync = false;
+            }
+            else
             {
                 double midRoundSyncTimeOut = uniqueEvents.Count / MaxEventsPerWrite * server.UpdateInterval.TotalSeconds;
                 midRoundSyncTimeOut = Math.Max(5.0f, midRoundSyncTimeOut * 1.5f);
@@ -257,11 +266,6 @@ namespace Barotrauma.Networking
                 client.NeedsMidRoundSync = true;
                 client.MidRoundSyncTimeOut = Timing.TotalTime + midRoundSyncTimeOut;
             }
-            else
-            {
-                client.UnreceivedEntityEventCount = 0;
-                client.NeedsMidRoundSync = false;
-            }
         }
 
         /// <summary>