diff --git a/homeassistant/components/mqtt/config_flow.py b/homeassistant/components/mqtt/config_flow.py
index 5f0984e9b9f..7fe01e9a890 100644
--- a/homeassistant/components/mqtt/config_flow.py
+++ b/homeassistant/components/mqtt/config_flow.py
@@ -1267,13 +1267,9 @@ class MQTTSubentryFlowHandler(ConfigSubentryFlow):
             reconfig=True,
         )
         if user_input is not None:
-            merged_user_input, errors = validate_user_input(
-                user_input, MQTT_DEVICE_PLATFORM_FIELDS
-            )
+            _, errors = validate_user_input(user_input, MQTT_DEVICE_PLATFORM_FIELDS)
             if not errors:
-                self._subentry_data[CONF_DEVICE] = cast(
-                    MqttDeviceData, merged_user_input
-                )
+                self._subentry_data[CONF_DEVICE] = cast(MqttDeviceData, user_input)
                 if self.source == SOURCE_RECONFIGURE:
                     return await self.async_step_summary_menu()
                 return await self.async_step_entity()
diff --git a/tests/components/mqtt/test_config_flow.py b/tests/components/mqtt/test_config_flow.py
index 2635263ae8e..c94d692b374 100644
--- a/tests/components/mqtt/test_config_flow.py
+++ b/tests/components/mqtt/test_config_flow.py
@@ -2908,6 +2908,10 @@ async def test_subentry_configflow(
         iter(config_subentries_data["components"].values())
     )
 
+    subentry_device_data = next(iter(config_entry.subentries.values())).data["device"]
+    for option, value in mock_device_user_input.items():
+        assert subentry_device_data[option] == value
+
     await hass.async_block_till_done()