Hi,
Can you please provide more details on your implementation?
As per current documentation available at docs.microsoft.com/.../nav-menu-module
The navigation menu module can be added to the header module of a page. In the Fabrikam theme, the navigation menu shows two levels by default. In the Starter theme, the navigation menu shows three levels by default.
To change to the number of levels, a view extension is required on the theme.
Best regards,
Dragos