There is something missing in DataObjects-as-Pages/javascript/SiteTreeURLSegmentField_modeladmin.js
If you load SiteTreeURLSegmentField.js you can edit the URL-Segment again.
If you uncomment Requirements::block(CMS_DIR . '/javascript/SiteTreeURLSegmentField.js'); in DataObjects-as-Pages/Code/Decorators/DataObjectAsPageLeftAndMain.php it will work.
There is something missing in DataObjects-as-Pages/javascript/SiteTreeURLSegmentField_modeladmin.js
If you load SiteTreeURLSegmentField.js you can edit the URL-Segment again.
If you uncomment Requirements::block(CMS_DIR . '/javascript/SiteTreeURLSegmentField.js'); in DataObjects-as-Pages/Code/Decorators/DataObjectAsPageLeftAndMain.php it will work.