Fix disposal units flushing too soon after power-on (#32314)

* Don't flush disposals immediately on power-on

* Update Content.Server/Disposal/Unit/EntitySystems/DisposalUnitSystem.cs

---------

Co-authored-by: Eoin Mcloughlin <helloworld@eoinrul.es>
Co-authored-by: metalgearsloth <31366439+metalgearsloth@users.noreply.github.com>

Content.Server/Disposal/Unit/EntitySystems/DisposalUnitSystem.cs

@@ -320,9 +320,10 @@ public sealed class DisposalUnitSystem : SharedDisposalUnitSystem
             return;
         }
 
-        if (component.Engaged && !TryFlush(uid, component))
+        if (component.Engaged)
         {
-            QueueAutomaticEngage(uid, component);
+            // Run ManualEngage to recalculate a new flush time
+            ManualEngage(uid, component);
         }
     }