diff --git a/akka-stream/src/main/mima-filters/2.6.11.backwards.excludes/29966-sub-inlet-outlet-memleak.backward.excludes b/akka-stream/src/main/mima-filters/2.6.11.backwards.excludes/29966-sub-inlet-outlet-memleak.backward.excludes
new file mode 100644
index 0000000000..5e4ed2b7a2
--- /dev/null
+++ b/akka-stream/src/main/mima-filters/2.6.11.backwards.excludes/29966-sub-inlet-outlet-memleak.backward.excludes
@@ -0,0 +1,7 @@
+# small optimization of ConcurrentAsyncCallback internal state
+ProblemFilters.exclude[MissingClassProblem]("akka.stream.stage.GraphStageLogic$ConcurrentAsyncCallback$Event$")
+ProblemFilters.exclude[MissingClassProblem]("akka.stream.stage.GraphStageLogic$ConcurrentAsyncCallback$State")
+ProblemFilters.exclude[MissingClassProblem]("akka.stream.stage.GraphStageLogic$ConcurrentAsyncCallback$Pending$")
+ProblemFilters.exclude[MissingClassProblem]("akka.stream.stage.GraphStageLogic$ConcurrentAsyncCallback$Event")
+ProblemFilters.exclude[MissingClassProblem]("akka.stream.stage.GraphStageLogic$ConcurrentAsyncCallback$Initialized$")
+ProblemFilters.exclude[MissingClassProblem]("akka.stream.stage.GraphStageLogic$ConcurrentAsyncCallback$Pending")
\ No newline at end of file
diff --git a/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala b/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
index ce0989411a..5fe7550abf 100644
--- a/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
+++ b/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
@@ -272,6 +272,22 @@ object GraphStageLogic {
   private[stream] val NoPromise: Promise[Done] = Promise.successful(Done)
 }
 
+/**
+ * INTERNAL API
+ */
+@InternalApi
+private[akka] object ConcurrentAsyncCallbackState {
+  sealed trait State[+E]
+  // waiting for materialization completion or during dispatching of initially queued events
+  final case class Pending[E](pendingEvents: List[Event[E]]) extends State[E]
+  // stream is initialized and so no threads can just send events without any synchronization overhead
+  case object Initialized extends State[Nothing]
+  // Event with feedback promise
+  final case class Event[E](e: E, handlingPromise: Promise[Done])
+
+  val NoPendingEvents = Pending[Nothing](Nil)
+}
+
 /**
  * Represents the processing logic behind a [[GraphStage]]. Roughly speaking, a subclass of [[GraphStageLogic]] is a
  * collection of the following parts:
@@ -1174,18 +1190,8 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
    * "Real world" calls of [[invokeWithFeedback]] always return failed promises for `Completed` state
    */
   private final class ConcurrentAsyncCallback[T](handler: T => Unit) extends AsyncCallback[T] {
-
-    sealed trait State
-    // waiting for materialization completion or during dispatching of initially queued events
-    // - can't be final because of SI-4440
-    private case class Pending(pendingEvents: List[Event]) extends State
-    // stream is initialized and so no threads can just send events without any synchronization overhead
-    private case object Initialized extends State
-    // Event with feedback promise - can't be final because of SI-4440
-    private case class Event(e: T, handlingPromise: Promise[Done])
-
-    private[this] val NoPendingEvents = Pending(Nil)
-    private[this] val currentState = new AtomicReference[State](NoPendingEvents)
+    import ConcurrentAsyncCallbackState._
+    private[this] val currentState = new AtomicReference[State[T]](NoPendingEvents)
 
     // is called from the owning [[GraphStage]]
     @tailrec
@@ -1240,7 +1246,7 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
 
         case list @ Pending(l) =>
           // not started yet
-          if (!currentState.compareAndSet(list, Pending(Event(event, promise) :: l)))
+          if (!currentState.compareAndSet(list, Pending[T](Event[T](event, promise) :: l)))
             invokeWithPromise(event, promise)
       }
 
@@ -1435,9 +1441,11 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
         case OnComplete =>
           closed = true
           handler.onUpstreamFinish()
+          GraphStageLogic.this.completedOrFailed(this)
         case OnError(ex) =>
           closed = true
           handler.onUpstreamFailure(ex)
+          GraphStageLogic.this.completedOrFailed(this)
       }
     }.invoke _)
 
@@ -1513,6 +1521,7 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
           available = false
           closed = true
           handler.onDownstreamFinish(cause)
+          GraphStageLogic.this.completedOrFailed(this)
         }
     }