diff --git a/bambulabs_api/mqtt_client.py b/bambulabs_api/mqtt_client.py
index d2b06bf..ec22a64 100644
--- a/bambulabs_api/mqtt_client.py
+++ b/bambulabs_api/mqtt_client.py
@@ -57,6 +57,25 @@ def is_valid_gcode(line: str):
     return True
 
 
+def _deep_merge(target: dict[str, Any], updates: dict[str, Any]) -> None:
+    """
+    Recursively merge updates into target, in place
+
+    Nested dictionaries are merged key by key, so an incremental report does
+    not drop sibling fields it did not mention. Lists and scalars are replaced.
+
+    Args:
+        target (dict[str, Any]): The cached data, modified in place
+        updates (dict[str, Any]): The newly received data
+    """
+    for key, value in updates.items():
+        existing = target.get(key)
+        if isinstance(existing, dict) and isinstance(value, dict):
+            _deep_merge(existing, value)
+        else:
+            target[key] = value
+
+
 class PrinterMQTTClient:
     """
     Printer class for handling MQTT communication with the printer
@@ -195,10 +214,7 @@ def _on_message(
         self.on_message_handler(self, client, userdata, msg)
 
     def manual_update(self, doc: dict[str, Any]) -> None:
-        for k, v in doc.items():
-            if k not in self._data:
-                self._data[k] = {}
-            self._data[k] |= v
+        _deep_merge(self._data, doc)
         logger.debug(self._data)
 
         firmware_version = self.firmware_version()
diff --git a/tests/test_mqtt_client.py b/tests/test_mqtt_client.py
index 295f637..db4889f 100644
--- a/tests/test_mqtt_client.py
+++ b/tests/test_mqtt_client.py
@@ -64,3 +64,45 @@ def test_nozzle_diameter():
 def test_get_firmware():
     assert mqtt.firmware_version() == "01.07.00.00"
     assert mqtt.printer_info.firmware_version == "01.07.00.00"
+
+
+def test_incremental_update_preserves_sibling_fields():
+    """A partial report must not wipe fields it does not mention."""
+    mqtt_ = bl.PrinterMQTTClient(hostname="", access="", printer_serial="")
+    mqtt_.manual_update(
+        {
+            "print": {
+                "gcode_state": "RUNNING",
+                "ams": {
+                    "ams_exist_bits": "1",
+                    "tray_exist_bits": "f",
+                    "ams": [{"id": "0", "tray": [{"id": "0"}]}],
+                },
+            },
+        }
+    )
+    # An incremental report mentioning only part of the ams object
+    mqtt_.manual_update(
+        {
+            "print": {
+                "ams": {
+                    "ams": [{"id": "0", "tray": [{"id": "0", "tray_color": "FF0000FF"}]}],
+                },
+            },
+        }
+    )
+
+    ams = mqtt_._data["print"]["ams"]  # noqa: SLF001
+    assert ams["ams_exist_bits"] == "1", "sibling field dropped by partial update"
+    assert ams["tray_exist_bits"] == "f", "sibling field dropped by partial update"
+    assert ams["ams"][0]["tray"][0]["tray_color"] == "FF0000FF", "update not applied"
+    assert mqtt_._data["print"]["gcode_state"] == "RUNNING"  # noqa: SLF001
+
+
+def test_update_replaces_scalars_and_lists():
+    """Lists and scalars are replaced, not merged."""
+    mqtt_ = bl.PrinterMQTTClient(hostname="", access="", printer_serial="")
+    mqtt_.manual_update({"print": {"gcode_state": "RUNNING", "s_obj": [1, 2]}})
+    mqtt_.manual_update({"print": {"gcode_state": "FINISH", "s_obj": [9]}})
+    assert mqtt_._data["print"]["gcode_state"] == "FINISH"  # noqa: SLF001
+    assert mqtt_._data["print"]["s_obj"] == [9]  # noqa: SLF001