Fixes issue when there are zero instructions in the db table and we've never synced before, in this case it will always cold boot because it will never saved the last synced, in this scenario also the indexes will not build because we've cleared the collection before they can run.

src/Umbraco.Core/Sync/DatabaseServerMessenger.cs

@@ -171,9 +171,11 @@ namespace Umbraco.Core.Sync
                     // go get the last id in the db and store it
                     // note: do it BEFORE initializing otherwise some instructions might get lost
                     // when doing it before, some instructions might run twice - not an issue
-                    var lastId = _appContext.DatabaseContext.Database.ExecuteScalar<int>("SELECT MAX(id) FROM umbracoCacheInstruction");
-                    if (lastId > 0)
-                        SaveLastSynced(lastId);
+                    var maxId = _appContext.DatabaseContext.Database.ExecuteScalar<int>("SELECT MAX(id) FROM umbracoCacheInstruction");
+
+                    //if there is a max currently, or if we've never synced
+                    if (maxId > 0 || _lastId < 0)
+                        SaveLastSynced(maxId);
 
                     // execute initializing callbacks
                     if (_options.InitializingCallbacks != null)

src/Umbraco.Web/WebBootManager.cs

@@ -551,9 +551,6 @@ namespace Umbraco.Web
             // return
             foreach (var index in indexes)
                 yield return index;
-
-            // and clear
-            IndexesToRebuild.Clear();
         }