Prevent a sub-menu added under new CPT

Hello… I am using CPT UI and CPT Auto Menu plugins. I run into a problem like below.

Steps:
1) create a new CPT with CPT UI, say Newsletters
2) create a page called Newsletters
3) assign page Newsletters as a menu item to an existing menu (ie. appearance > menu)
4) use CPT Auto Menu to assign CPT Newsletters (ie. created in step 1) to page Newsletters

I can add the new Newsletters via wp-admin but then I found there are some newly added Newsletters automatically added to the Newsletters menu item (ie. created in step 3) as sub-menu.
If I add more newsletters, there will be many sub-menu items added.
I want to prevent the sub-menus added under new CPT.

Can anyone help please? Many thanks!