diff --git a/third_party/dawn/src/dawn/native/d3d12/CommandBufferD3D12.cpp b/third_party/dawn/src/dawn/native/d3d12/CommandBufferD3D12.cpp
--- a/third_party/dawn/src/dawn/native/d3d12/CommandBufferD3D12.cpp
+++ b/third_party/dawn/src/dawn/native/d3d12/CommandBufferD3D12.cpp
@@ -1366,12 +1366,13 @@
                 WriteBufferCmd* write = mCommands.NextCommand<WriteBufferCmd>();
                 const uint64_t offset = write->offset;
                 const uint64_t size = write->size;
+                // Always consume the kAdditionalData marker to keep the iterator in sync.
+                uint8_t* data = mCommands.NextData<uint8_t>(size);
                 if (size == 0) {
                     continue;
                 }

                 Buffer* dstBuffer = ToBackend(write->buffer.Get());
-                uint8_t* data = mCommands.NextData<uint8_t>(size);

                 DAWN_TRY(device->GetDynamicUploader()->WithUploadReservation(
                     size, kCopyBufferToBufferOffsetAlignment,
diff --git a/third_party/dawn/src/dawn/native/metal/CommandBufferMTL.mm b/third_party/dawn/src/dawn/native/metal/CommandBufferMTL.mm
--- a/third_party/dawn/src/dawn/native/metal/CommandBufferMTL.mm
+++ b/third_party/dawn/src/dawn/native/metal/CommandBufferMTL.mm
@@ -1508,12 +1508,13 @@
                 WriteBufferCmd* write = mCommands.NextCommand<WriteBufferCmd>();
                 const uint64_t offset = write->offset;
                 const uint64_t size = write->size;
+                // Always consume the kAdditionalData marker to keep the iterator in sync.
+                uint8_t* data = mCommands.NextData<uint8_t>(size);
                 if (size == 0) {
                     continue;
                 }

                 Buffer* dstBuffer = ToBackend(write->buffer.Get());
-                uint8_t* data = mCommands.NextData<uint8_t>(size);
                 Device* device = ToBackend(GetDevice());

                 DAWN_TRY(device->GetDynamicUploader()->WithUploadReservation(
diff --git a/third_party/dawn/src/dawn/native/opengl/CommandBufferGL.cpp b/third_party/dawn/src/dawn/native/opengl/CommandBufferGL.cpp
--- a/third_party/dawn/src/dawn/native/opengl/CommandBufferGL.cpp
+++ b/third_party/dawn/src/dawn/native/opengl/CommandBufferGL.cpp
@@ -1136,12 +1136,13 @@
                 WriteBufferCmd* write = mCommands.NextCommand<WriteBufferCmd>();
                 uint64_t offset = write->offset;
                 uint64_t size = write->size;
+                // Always consume the kAdditionalData marker to keep the iterator in sync.
+                uint8_t* data = mCommands.NextData<uint8_t>(size);
                 if (size == 0) {
                     continue;
                 }

                 Buffer* dstBuffer = ToBackend(write->buffer.Get());
-                uint8_t* data = mCommands.NextData<uint8_t>(size);
                 DAWN_TRY(dstBuffer->EnsureDataInitializedAsDestination(offset, size));

                 DAWN_GL_TRY(gl, BindBuffer(GL_ARRAY_BUFFER, dstBuffer->GetHandle()));
diff --git a/third_party/dawn/src/dawn/native/vulkan/CommandBufferVk.cpp b/third_party/dawn/src/dawn/native/vulkan/CommandBufferVk.cpp
--- a/third_party/dawn/src/dawn/native/vulkan/CommandBufferVk.cpp
+++ b/third_party/dawn/src/dawn/native/vulkan/CommandBufferVk.cpp
@@ -1300,12 +1300,13 @@
                 WriteBufferCmd* write = mCommands.NextCommand<WriteBufferCmd>();
                 const uint64_t offset = write->offset;
                 const uint64_t size = write->size;
+                // Always consume the kAdditionalData marker to keep the iterator in sync.
+                uint8_t* data = mCommands.NextData<uint8_t>(size);
                 if (size == 0) {
                     continue;
                 }

                 Buffer* dstBuffer = ToBackend(write->buffer.Get());
-                uint8_t* data = mCommands.NextData<uint8_t>(size);

                 DAWN_TRY(device->GetDynamicUploader()->WithUploadReservation(
                     size, kCopyBufferToBufferOffsetAlignment,