Sub-menu is not getting added or linked to menu page and looks different/separated from other sub-menus, while disc authoring.
When you remove all or any one of navigation buttons ('Next' 'Main' and 'Previous') manually from a sub-menu, such issue occurs.
There is no need to remove default navigation buttons('Next' 'Main' and 'Previous').
To resolve the issue, remove the problematic sub-menu from menu list then add it back to the project and continue without removing navigation buttons, navigation buttons should not be removed manually from a linked sub-menu.
Note: Normally just clicking between two menus(on the bank space), menu gets added/linked into the menu page.