Fixes #8472

Marlin/ultralcd.cpp

       #if EXTRUDERS == 1
         MENU_MULTIPLIER_ITEM_EDIT_CALLBACK(float43, MSG_FILAMENT_DIAM, &planner.filament_size[0], 1.5, 3.25, planner.calculate_volumetric_multipliers);
       #else // EXTRUDERS > 1
+        MENU_MULTIPLIER_ITEM_EDIT_CALLBACK(float43, MSG_FILAMENT_DIAM, &planner.filament_size[active_extruder], 1.5, 3.25, planner.calculate_volumetric_multipliers);
         MENU_MULTIPLIER_ITEM_EDIT_CALLBACK(float43, MSG_FILAMENT_DIAM MSG_DIAM_E1, &planner.filament_size[0], 1.5, 3.25, planner.calculate_volumetric_multipliers);
         MENU_MULTIPLIER_ITEM_EDIT_CALLBACK(float43, MSG_FILAMENT_DIAM MSG_DIAM_E2, &planner.filament_size[1], 1.5, 3.25, planner.calculate_volumetric_multipliers);
         #if EXTRUDERS > 2