Menu items disable and weight resets on save
I think I have discovered this issue with Drupal 8 too.

I have the default Menu navigation from the Bootstrap theme, which I am trying to add a menu item for an "About me" page.  When I edit the menu item link and save with correct weight and Enabled set, the menu item is ticked. However, when I click the Save button on the main Menu page, the About Me item is disabled. Two items remain ticked - the Home menu and a View.

There seems to be a bug in the global Save button for the Menu. I guess the workaround is to modify menu items by clicking Edit, rather than directly on the menu page.