some fixes

python_app/hardware_full/multi_device_service.py

@@ -76,11 +76,14 @@ class MultiDeviceLibreVnaService:
                 slave_serial_numbers=self.slave_serials,
                 force_external_reference=self.force_external_reference,
             )
-        except Exception:
-            if self.backend_mode == "native":
-                raise
-            self._using_mock_backend = True
-            self._controller = None
+        except Exception as exc:
+            # Never silently latch to synthetic data: a deployed appliance must wait
+            # for the real device, not record fakes. Synthetic data requires an
+            # explicit backend_mode='mock' (selected in __post_init__); both 'auto'
+            # and 'native' re-raise so the producer's wait-for-device retry keeps
+            # trying until the hardware appears.
+            logger.warning("Multi-device open failed (backend_mode=%s): %s", self.backend_mode, exc)
+            raise
 
     def close(self) -> None:
         """Close native device transports; never raises.