diff --git a/com.unity.netcode.gameobjects/CHANGELOG.md b/com.unity.netcode.gameobjects/CHANGELOG.md
index 9829470469..75f9b9f411 100644
--- a/com.unity.netcode.gameobjects/CHANGELOG.md
+++ b/com.unity.netcode.gameobjects/CHANGELOG.md
@@ -22,6 +22,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Issue where bits written with `BitWriter` were lost when a later `BitWriter.TryBeginWriteBits` call in the same bitwise context grew the `FastBufferWriter`. The bytes written earlier in that context were replaced with zeros or uninitialized memory. (#4225)
 - Issue where a `FastBufferReader` created with an offset and the default length of -1 read past the end of its source. The `ArraySegment<byte>` overload that takes an offset now applies it from the start of the segment. (#4221)
 - Issue where a client synchronized while a `NetworkList<T>` still had unsent changes applied those changes twice, resulting in duplicate entries and index-based events landing on the wrong elements on that client. (#4215)
 - Issue where showing a `NetworkObject` with `NetworkShow` while its `NetworkVariable` updates were held back by `NetworkVariableUpdateTraits` stopped those pending changes from being sent until the variable was modified again. (#4215)
diff --git a/com.unity.netcode.gameobjects/Runtime/Serialization/FastBufferWriter.cs b/com.unity.netcode.gameobjects/Runtime/Serialization/FastBufferWriter.cs
index 343822fe32..3d122aa671 100644
--- a/com.unity.netcode.gameobjects/Runtime/Serialization/FastBufferWriter.cs
+++ b/com.unity.netcode.gameobjects/Runtime/Serialization/FastBufferWriter.cs
@@ -204,7 +204,8 @@ internal unsafe void Grow(int additionalSizeRequired)
 #if DEBUG
             UnsafeUtility.MemSet(newBuffer, 0, newSize);
 #endif
-            UnsafeUtility.MemCpy(newBuffer, Handle->BufferPointer, Length);
+            // Not Length: bytes written in an open bitwise context are not committed to Position until it is disposed.
+            UnsafeUtility.MemCpy(newBuffer, Handle->BufferPointer, Handle->Capacity);
             if (Handle->BufferGrew)
             {
                 UnsafeUtility.Free(Handle->BufferPointer, Handle->Allocator);
diff --git a/com.unity.netcode.gameobjects/Tests/Editor/Serialization/BitWriterTests.cs b/com.unity.netcode.gameobjects/Tests/Editor/Serialization/BitWriterTests.cs
index b4a84ff710..c98138a77c 100644
--- a/com.unity.netcode.gameobjects/Tests/Editor/Serialization/BitWriterTests.cs
+++ b/com.unity.netcode.gameobjects/Tests/Editor/Serialization/BitWriterTests.cs
@@ -329,5 +329,30 @@ public unsafe void TestWritingBitsThrowsIfTryBeginWriteNotCalled()
 
             }
         }
+
+        [Test]
+        public void TestTryBeginWriteBitsGrowingBufferKeepsBitwiseWrites([Values] bool writeLeadingByte)
+        {
+            var writer = new FastBufferWriter(writeLeadingByte ? 2 : 1, Allocator.Temp, 16);
+            using (writer)
+            {
+                if (writeLeadingByte)
+                {
+                    writer.WriteByteSafe(0x11);
+                }
+
+                using (var bitWriter = writer.EnterBitwiseContext())
+                {
+                    Assert.IsTrue(bitWriter.TryBeginWriteBits(8));
+                    bitWriter.WriteBits(0xAB, 8);
+                    // Grows the buffer while 0xAB is still uncommitted to the writer.
+                    Assert.IsTrue(bitWriter.TryBeginWriteBits(16));
+                    bitWriter.WriteBits(0xCDEFUL, 16);
+                }
+
+                var expected = writeLeadingByte ? new byte[] { 0x11, 0xAB, 0xEF, 0xCD } : new byte[] { 0xAB, 0xEF, 0xCD };
+                CollectionAssert.AreEqual(expected, writer.ToArray());
+            }
+        }
     }
 }