diff --git a/driver-core/src/main/com/mongodb/internal/connection/AsynchronousChannelStream.java b/driver-core/src/main/com/mongodb/internal/connection/AsynchronousChannelStream.java
index 89396dae5d..ec72cc777d 100644
--- a/driver-core/src/main/com/mongodb/internal/connection/AsynchronousChannelStream.java
+++ b/driver-core/src/main/com/mongodb/internal/connection/AsynchronousChannelStream.java
@@ -41,6 +41,8 @@
 import static com.mongodb.internal.thread.InterruptionUtil.interruptAndCreateMongoInterruptedException;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 
+import org.bson.ByteBufNIO;
+
 /**
  * <p>This class is not part of the public API and may be removed or changed at any time</p>
  */
@@ -174,7 +176,11 @@ private void pipeOneBuffer(final AsyncWritableByteChannelAdapter byteChannel, fi
         byteChannel.write(byteBuffer.asNIO(), operationContext, new AsyncCompletionHandler<Void>() {
             @Override
             public void completed(@Nullable final Void t) {
-                if (byteBuffer.hasRemaining()) {
+                boolean isNioBuf = byteBuffer instanceof ByteBufNIO;
+                // in a rare case when ByteBufNIO was already released
+                // the call to `hasRemaining` may throw an NPE
+                // so we add this new if condition to catch this case
+                if ((isNioBuf && byteBuffer.asNIO() != null && byteBuffer.hasRemaining()) || byteBuffer.hasRemaining()) {
                     byteChannel.write(byteBuffer.asNIO(), operationContext, this);
                 } else {
                     outerHandler.completed(null);
diff --git a/driver-reactive-streams/src/test/functional/com/mongodb/reactivestreams/client/CrudProseTest.java b/driver-reactive-streams/src/test/functional/com/mongodb/reactivestreams/client/CrudProseTest.java
index 07afb94c8d..6dd0ed7c46 100644
--- a/driver-reactive-streams/src/test/functional/com/mongodb/reactivestreams/client/CrudProseTest.java
+++ b/driver-reactive-streams/src/test/functional/com/mongodb/reactivestreams/client/CrudProseTest.java
@@ -28,4 +28,10 @@ final class CrudProseTest extends com.mongodb.client.CrudProseTest {
     protected MongoClient createMongoClient(final MongoClientSettings.Builder mongoClientSettingsBuilder) {
         return new SyncMongoClient(mongoClientSettingsBuilder);
     }
+
+    @Override
+    protected MongoClient createMongoClientWithExtendedCloseWait(final MongoClientSettings.Builder mongoClientSettingsBuilder,
+            final int waitIterations) {
+        return new SyncMongoClient(mongoClientSettingsBuilder, null, waitIterations);
+    }
 }
diff --git a/driver-reactive-streams/src/test/functional/com/mongodb/reactivestreams/client/syncadapter/SyncMongoClient.java b/driver-reactive-streams/src/test/functional/com/mongodb/reactivestreams/client/syncadapter/SyncMongoClient.java
index 348bef110f..ae3825af6b 100644
--- a/driver-reactive-streams/src/test/functional/com/mongodb/reactivestreams/client/syncadapter/SyncMongoClient.java
+++ b/driver-reactive-streams/src/test/functional/com/mongodb/reactivestreams/client/syncadapter/SyncMongoClient.java
@@ -152,7 +152,12 @@ public SyncMongoClient(final MongoClientSettings.Builder builder) {
     }
 
     public SyncMongoClient(final MongoClientSettings.Builder builder, @Nullable final MongoDriverInformation mongoDriverInformation) {
-        this.connectionPoolCounter = new ConnectionPoolCounter();
+        this(builder, mongoDriverInformation, ConnectionPoolCounter.DEFAULT_MAX_ITERATIONS);
+    }
+
+    public SyncMongoClient(final MongoClientSettings.Builder builder, @Nullable final MongoDriverInformation mongoDriverInformation,
+            final int connectionCloseWaitIterations) {
+        this.connectionPoolCounter = new ConnectionPoolCounter(connectionCloseWaitIterations);
         builder.applyToConnectionPoolSettings(b -> b.addConnectionPoolListener(connectionPoolCounter));
         this.wrapped = MongoClients.create(builder.build(), mongoDriverInformation);
         this.delegate = new SyncMongoCluster(wrapped);
@@ -345,7 +350,13 @@ public void appendMetadata(final MongoDriverInformation mongoDriverInformation)
     }
 
     static class ConnectionPoolCounter implements ConnectionPoolListener {
+        static final int DEFAULT_MAX_ITERATIONS = 10; // 10 * 200ms = 2 seconds
         private final AtomicInteger activeConnections = new AtomicInteger(0);
+        private final int maxIterations;
+
+        ConnectionPoolCounter(final int maxIterations) {
+            this.maxIterations = maxIterations;
+        }
 
         @Override
         public void connectionCheckedOut(final ConnectionCheckedOutEvent event) {
@@ -361,7 +372,10 @@ protected void assertConnectionsClosed() {
             int activeConnectionsCount = activeConnections.get();
             boolean connectionsClosed = activeConnectionsCount == 0;
             int counter = 0;
-            while (counter < 10 && !connectionsClosed) {
+            // Wait up to maxIterations (default 2 seconds) for connections to close.
+            // This allows time for long-running operations (like bulk writes with many errors)
+            // to complete their cleanup after cancellation.
+            while (counter < maxIterations && !connectionsClosed) {
                 activeConnectionsCount = activeConnections.get();
                 connectionsClosed = activeConnectionsCount == 0;
                 if (!connectionsClosed) {
@@ -369,7 +383,6 @@ protected void assertConnectionsClosed() {
                     counter++;
                 }
             }
-
             Assertions.assertTrue(activeConnectionsCount == 0,
                     format("Expected all connections to be closed after closing the client. %n"
                             + "The connection pool listener reports '%d' open connections.", activeConnectionsCount));
diff --git a/driver-sync/src/test/functional/com/mongodb/client/CrudProseTest.java b/driver-sync/src/test/functional/com/mongodb/client/CrudProseTest.java
index d269a3cad5..efc0049936 100644
--- a/driver-sync/src/test/functional/com/mongodb/client/CrudProseTest.java
+++ b/driver-sync/src/test/functional/com/mongodb/client/CrudProseTest.java
@@ -243,9 +243,12 @@ protected void testBulkWriteCollectsWriteConcernErrorsAcrossBatches() throws Int
     protected void testBulkWriteHandlesWriteErrorsAcrossBatches(final boolean ordered) {
         assumeTrue(serverVersionAtLeast(8, 0));
         TestCommandListener commandListener = new TestCommandListener();
-        try (MongoClient client = createMongoClient(getMongoClientSettingsBuilder()
+        // Use longer timeout (300 iterations = 60 seconds) as this test involves many write errors
+        // and the async cleanup takes longer than the default 2 seconds
+        final int waitIterations = 300;
+        try (MongoClient client = createMongoClientWithExtendedCloseWait(getMongoClientSettingsBuilder()
                 .retryWrites(false)
-                .addCommandListener(commandListener))) {
+                .addCommandListener(commandListener), waitIterations)) {
             int maxWriteBatchSize = droppedDatabase(client).runCommand(new Document("hello", 1)).getInteger("maxWriteBatchSize");
             Document document = new Document("_id", 1);
             MongoCollection<Document> collection = droppedCollection(client, Document.class);
@@ -564,6 +567,16 @@ protected MongoClient createMongoClient(final MongoClientSettings.Builder mongoC
         return MongoClients.create(mongoClientSettingsBuilder.build());
     }
 
+    /**
+     * Should be ignored by sync driver.
+     * The only reason for this method to exist is
+     * to allow async driver to extend the close wait time
+     */
+    protected MongoClient createMongoClientWithExtendedCloseWait(final MongoClientSettings.Builder mongoClientSettingsBuilder,
+            final int waitIterations) {
+        return createMongoClient(mongoClientSettingsBuilder);
+    }
+
     private <TDocument> MongoCollection<TDocument> droppedCollection(final MongoClient client, final Class<TDocument> documentClass) {
         return droppedDatabase(client).getCollection(NAMESPACE.getCollectionName(), documentClass);
     }