As a temporary workaround until WordPress fixes this issue you can add new pages with the following steps:
- Add page like normal (from the options on the left sidebar pages/categories etc...), page will appear at bottom of menu.
- Save the menu (with page at the bottom still)
- When the menu saves/page refreshes, scroll to the bottom where the new menu item is located, use the top-right dropdown arrow to reveal the extra options, use the links such as 'up one, down one, to top' to move the menu items.
This is a really terrible way to manage the menu (especially a large one) but it's working in the meantime. The key is to save the menu before you try to use the links to move it around - they only work on saved menu items, not news ones added to the menu without a save.
You can also manage the position of existing pages using those links.