I’m actually having the same exact problem. I’ve setup a test site with a clean install of WP4.0 and nothing else installed (literally no plugins or themes) and it behaves as described above.
I’m happy to create an account for support to login and take a peak if you’d like to see if happening for yourselves.
This, by the way, is running on a Windows host. I’d be curious to know if everyone else having this issue is on a Windows host.
Let me know how I can help… this is intensely frustrating.
Dan