Run the Flow Rate Fine Pass on the Extruder the Coarse Pass Used

When the fine pass starts from the coarse page the filament comes from
the cached preset, but the extruder was resolved from whatever the
preset page currently has selected. After a restart or reconnect, or if
the selection was changed, that no longer matches and the lookup falls
back to the per-AMS map, which with a Filament Track Switch pins every
AMS to the main extruder. Take the extruder from the cache in that path;
it already records the one the coarse pass was sent with.

src/slic3r/GUI/CalibrationWizard.cpp

@@ -1309,6 +1309,14 @@ void FlowRateWizard::on_cali_start(CaliPresetStage stage, float cali_value, Flow
             get_tray_ams_and_slot_id(curr_obj, selected_filaments.begin()->first, calib_info.ams_id, calib_info.slot_id, selected_tray_id);
             calib_info.select_ams         = std::to_string(selected_tray_id);
             calib_info.extruder_id = preset_page->get_extruder_id_by_tray_id(selected_filaments.begin()->first);
+            if (from_page == FlowRatioCaliSource::FROM_COARSE_PAGE) {
+                // The fine pass must run on the extruder the coarse pass used. The cache records it;
+                // the preset page's selection may have changed or been reset since.
+                const auto cached = get_cached_selected_filament_for_multi_extruder(curr_obj);
+                const auto it     = cached.find(selected_filaments.begin()->first);
+                if (it != cached.end())
+                    calib_info.extruder_id = it->second.extruder_id;
+            }
             calib_info.extruder_type      = preset_page->get_extruder_type(calib_info.extruder_id);
             calib_info.nozzle_volume_type = preset_page->get_nozzle_volume_type(calib_info.extruder_id);
             calib_info.nozzle_diameter    = preset_page->get_nozzle_diameter(calib_info.extruder_id);