After updating core to 10.4, the menu widget started showing "Not in menu" summary item, even though the node certainly is in the menu.
Update core to 10.4.something.
Currently the MenuLinkWidget.php includes its own version (library menu_link.form) of the JS for adding the summary (The menu item title, or the "Not in menu" text), most likely due to the field name being different than in core. The core library from Menu UI is drupal.menu_ui.
On the MenuLinkWidget.php, both of the libraries mentioned above are added to the elements. To my understanding, as menu_link is providing its own library with the same functionalities, the core library is unnecessary here. And for one reason or another, after the core update, it's being triggered after the menu_link's library.
The solution would be to remove the core's library from being included.
Active
2.0
Code
Attached is a patch for removing the core's library attaching, and only using the menu_link provided library instead.
Looks good to me also, since patches are deprecated providing with and MR.