fix labeling (#756)

Co-authored-by: Boisselet Alexandre (IFAT DC ATV SC D TE2) <Alexandre.Boisselet@infineon.com>

magpylib/_src/display/traces_utility.py

@@ -282,22 +282,23 @@ def get_flatten_objects_properties_recursive(
         style = get_style(subobj, default_settings, **kwargs)
         if style.label is None:
             style.label = str(type(subobj).__name__)
-        if parent_legendgroup is not None:
-            legendgroup = parent_legendgroup
-        else:
-            legendgroup = f"{subobj}"
+        legendgroup = f"{subobj}" if parent_legendgroup is None else parent_legendgroup
+        label = (
+            get_legend_label(subobj, style=style)
+            if parent_label is None
+            else parent_label
+        )
         if parent_color is not None and style.color is None:
             style.color = parent_color
         elif style.color is None:
             style.color = next(color_cycle)
         flat_objs[subobj] = {
             "legendgroup": legendgroup,
             "style": style,
-            "legendtext": parent_label,
+            "legendtext": label,
             "showlegend": parent_showlegend,
         }
         if isCollection:
-            label = get_legend_label(subobj, style=style)
             flat_objs.update(
                 get_flatten_objects_properties_recursive(
                     *subobj.children,