Description

• Delete all menus.
• Site Editor.
• Create Nav block and create a Menu adding 1 item.
• Save.
• Go back to Nav block and Select menu -> Create new.
• Add some items to this new Menu.
• Save.
• Notice how both menus are now marked as dirty.
• Saving causes the original / first menu to loose all its items.

Found by @getdave in #36828 (review)