Thanks for the project - confirmed, this is a bug.
The menu bar designer and the saved project data are fine; the problem is in the macOS runtime. On macOS the native menu bar is built directly against the operating system's menu system, and that code path currently never draws a menu item's Symbol (or Icon). The designer preview, menu bars on Windows and Linux, and context menus on every platform all show them correctly.
I've logged it and it's accepted for a future release: Menu bar items on macOS do not show their symbols and icons.