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 un-editable.
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.