When I review a question with a vote to close as duplicate, I see MathJaX formatting for the question body, but not in the tab that shows the proposed duplicate. Chrome 23.0.1271.17 beta in case it matters.
This seems to be a common issue in various less thoroughly tested parts of the Stack Exchange code base. For example, it also happens in the 10k flag review for any posts hidden behing the "show already flagged posts" link.
The fundamental cause of this bug is a combination of the following issues:
The proper way to fix this would, arguably, be for the Stack Exchange UI code to define its own "show a hidden element" method that was aware of these issues, and then systematically use that method instead of the plain jQuery
In the mean time, however, an alternative fix is to modify the jQuery