How is the main menu added to the viewport?
Is this running in multiplayer at all?
Dedicated server?
Is it in a HUD class and called there?
Is it your own HUD class?
A umg should be called in a HUD class which is the default for your menu “level” you would want a menu player controller.
In the actual game map you would want an I game controller that is different then the menu controller.
If even suggest a HUD class for in game so when the level loads it starts fresh.