Clear cached flag names on sync

Closes #1907

qt/aqt/flags.py

@@ -55,6 +55,10 @@ class FlagManager:
         self.mw.col.set_config("flagLabels", labels)
         gui_hooks.flag_label_did_change()
 
+    def require_refresh(self) -> None:
+        "Discard cached labels."
+        self._flags = None
+
     def _load_flags(self) -> None:
         labels = cast(dict[str, str], self.mw.col.get_config("flagLabels", {}))
         icon = ColoredIcon(path="icons:flag.svg", color=colors.DISABLED)

qt/aqt/main.py

@@ -953,6 +953,7 @@ title="{}" {}>{}</button>""".format(
 
     def _refresh_after_sync(self) -> None:
         self.toolbar.redraw()
+        self.flags.require_refresh()
 
     def _sync_collection_and_media(self, after_sync: Callable[[], None]) -> None:
         "Caller should ensure auth available."