The issue happens when trying to create a personalized list view to be shared on the Roadmap Details page. This also happens on all other Modern / New User Experience (MUX / UX) pages and List views.
When you create a view and another user marks it as the default, the view becomes uneditable.
When you try to set one of your views as the Default in Manage, the saved views are available and the favorite checkbox can be selected/deselected but the 'default' checkbox is greyed out.
You also cannot select/deselect the 'default' option on any existing list view. If the view is already checked as the default, you cannot make changes to that view.
If you try to edit a Default View and then Confirm, one of the below happens:
Release : 15.8 and higher
Grant the right View - Administer to the user to be able to save/edit default views. This is a new right that has been added in 15.8. Make sure you add it to all the administrator users, as it won't be added automatically.