I have a page with two WebDialogWindows on it. I have both windows set to modal and both to hidden until a button is clicked to show them.
When I open the first window, it is modal. However, when the 2nd one is opened it is not.
It does not matter in which order I open then, the 2nd one is alway not modal.
This has been fixed in the hotfix that was released last week.
I do not have a solution, but I can confirm this behavior since I am having the same problem.