Yes, I can also confirm this. I was really looking forward to using the new Expo mode, but it's currently not usable with multiple displays.
I'm using an HP ProBook 6450b with integrated Intel graphics. Laptop LCD is primary and external BENQ LCD (1920x1080) is secondary. When Expo is invoked, the secondary display goes dim, but nothing displays on it -- apps disappear and then reappear when a desktop or window is selected from the primary. It's almost as if the desktop on the external monitor displays overtop the app windows and, for whatever reason, isn't able to display the scaled versions of the apps overtop. (I have similar issues with Conky where it will display correctly on top of the desktop, but then clicking anywhere on the desktop causes the desktop to display overtop Conky, hiding it. I've only noticed this sort of behavior with Cinnamon.)
The new "only use workspaces on primary monitor" option doesn't seem to have any affect either way.
One thing that was interesting is that I re-started Firefox after having exiting it (it was originally running on the secondary monitor) and it automatically opened on the secondary monitor but in it's miniturized (scaled) form. It was quite finicky and I had to click on it several times before getting it to scale back to normal size. Might be a clue as to what's going on.
I really need this new Expo feature as the Scale option doesn't automatically handle apps running across virtual desktops (i.e. you need to switch to the desktop and then invoke Scale).
Clem, I'm really hoping there's a quick bug fix for this and would be more than happy to assist in troubleshooting or providing additional info. Thanks.