diff --git a/stream-tests/src/test/scala/org/apache/pekko/stream/impl/fusing/ActorGraphInterpreterSpec.scala b/stream-tests/src/test/scala/org/apache/pekko/stream/impl/fusing/ActorGraphInterpreterSpec.scala
index b6919b4f1e..5af288c961 100644
--- a/stream-tests/src/test/scala/org/apache/pekko/stream/impl/fusing/ActorGraphInterpreterSpec.scala
+++ b/stream-tests/src/test/scala/org/apache/pekko/stream/impl/fusing/ActorGraphInterpreterSpec.scala
@@ -436,5 +436,20 @@ class ActorGraphInterpreterSpec extends StreamSpec {
       done.future.futureValue // would throw on failure
     }
 
+    "continue working with subfused interpreters after initial shell reference is released" in {
+      val mat = Materializer(system)
+
+      // flatMapConcat triggers subfusing: the inner Source is materialized via
+      // SubFusingActorMaterializerImpl, which registers additional shells into
+      // the same ActorGraphInterpreter actor via registerShell.
+      // The actor must continue processing subfused shells correctly after
+      // the initial shell reference is released in preStart().
+      val result = Source(1 to 3)
+        .flatMapConcat(i => Source(List(i, i * 10)))
+        .runWith(Sink.seq)(mat)
+
+      result.futureValue should ===(Seq(1, 10, 2, 20, 3, 30))
+    }
+
   }
 }
diff --git a/stream/src/main/scala/org/apache/pekko/stream/impl/fusing/ActorGraphInterpreter.scala b/stream/src/main/scala/org/apache/pekko/stream/impl/fusing/ActorGraphInterpreter.scala
index 61eb9b8a29..d11aa15b37 100644
--- a/stream/src/main/scala/org/apache/pekko/stream/impl/fusing/ActorGraphInterpreter.scala
+++ b/stream/src/main/scala/org/apache/pekko/stream/impl/fusing/ActorGraphInterpreter.scala
@@ -729,7 +729,7 @@ import org.reactivestreams.Subscription
 /**
  * INTERNAL API
  */
-@InternalApi private[pekko] final class ActorGraphInterpreter(_initial: GraphInterpreterShell)
+@InternalApi private[pekko] final class ActorGraphInterpreter(private var _initial: GraphInterpreterShell)
     extends Actor
     with ActorLogging {
   import ActorGraphInterpreter._
@@ -742,7 +742,7 @@ import org.reactivestreams.Subscription
     try {
       currentLimit = shell.init(self, subFusingMaterializerImpl, enqueueToShortCircuit, currentLimit)
       if (GraphInterpreter.Debug)
-        println(s"registering new shell in ${_initial}\n  ${shell.toString.replace("\n", "\n  ")}")
+        println(s"registering new shell in ${shell}\n  ${shell.toString.replace("\n", "\n  ")}")
       if (shell.isTerminated) false
       else {
         activeInterpreters += shell
@@ -790,6 +790,9 @@ import org.reactivestreams.Subscription
 
   override def preStart(): Unit = {
     tryInit(_initial)
+    // Release reference to initial shell to avoid keeping it alive after it is shut down
+    // when the actor is still alive hosting other subfused interpreters
+    _initial = null
     if (activeInterpreters.isEmpty) context.stop(self)
     else if (shortCircuitBuffer ne null) shortCircuitBatch()
   }