I wrote a paragraph in another answer that I enclosed with parentheses. As I typed the closing parenthesis, I noticed that both the opening and closing parentheses disappeared in the preview! After submitting my answer, they appeared correctly in the post.
This appears to have been caused by the Markdown "footnote" syntax above the paragraph I was writing. Try this:
blah [blah] : http://example.com (foo bar)
The parentheses around
(foo bar) will disappear in the preview.