(This is using 2D, in Unity 5f1)
This might be a bug, I'm not sure.
In the settings manager, under Interface settings, Close Interactions with: Cursor Leaves Menus appears to have a different behaviour than described. For me my interaction menu doesn't close when the mouse leaves the menu, it closes when the mouse leaves the hotspot, as illustrated in the gif below.
The behaviour is the same using Unity UI or AC's menu system. Using Click Off Menu to close the menu works as intended, it's enough to click outside the menu, I don't have to click outside the hotspot for it to work.
Comments
The menu will close if it leaves BOTH the Hotspot and the Menu. But I do agree it's a problem in this case, and I will consider it in an update.