fix: server, host, or session owner not populating in-scene placed object table when started

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -17,6 +17,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Fixed issue where the server, host, or session owner would not populate the in-scene place `NetworkObject` table if the scene was loaded prior to starting the `NetworkManager`. (#3177)
 - Fixed issue where the `NetworkObjectIdHash` value could be incorrect when entering play mode while still in prefab edit mode with pending changes and using MPPM. (#3162)
 - Fixed issue where a sever only `NetworkManager` instance would spawn the actual `NetworkPrefab`'s `GameObject` as opposed to creating an instance of it. (#3160)
 - Fixed issue where only the session owner (as opposed to all clients) would handle spawning prefab overrides properly when using a distributed authority network topology. (#3160)

com.unity.netcode.gameobjects/Runtime/Spawning/NetworkSpawnManager.cs

@@ -1449,6 +1449,15 @@ internal void ServerSpawnSceneObjectsOnStartSweep()
                 }
             }
 
+            // Since we are spawing in-scene placed NetworkObjects for already loaded scenes,
+            // we need to add any in-scene placed NetworkObject to our tracking table 
+            var clearFirst = true;
+            foreach (var sceneLoaded in NetworkManager.SceneManager.ScenesLoaded)
+            {
+                NetworkManager.SceneManager.PopulateScenePlacedObjects(sceneLoaded.Value, clearFirst);
+                clearFirst = false;
+            }
+
             // Notify all in-scene placed NetworkObjects have been spawned
             foreach (var networkObject in networkObjectsToSpawn)
             {