Test post with "Hidden " as the subject line.
(EDIT) Turns out the quotation marks, in conjunction with the word Hidden caused the issue, somehow. Replacing the actual quotation marks with character code " allows the subject line to appear as expected. Using actual quotes, instead of the code, however, causes the subject line to disappear, thanks to some strange html shenanigans. For this particular post, the subject line is now actually "Hidden " rather than "Hidden ". (/EDIT)
(EDIT 2) Here is a companion post, which demonstrates the original issue of the disappearing subject line. (/EDIT 2)
(EDIT 3) Issue originally encountered here, for reference. (/EDIT 3)
(EDIT) Turns out the quotation marks, in conjunction with the word Hidden caused the issue, somehow. Replacing the actual quotation marks with character code " allows the subject line to appear as expected. Using actual quotes, instead of the code, however, causes the subject line to disappear, thanks to some strange html shenanigans. For this particular post, the subject line is now actually "Hidden " rather than "Hidden ". (/EDIT)
(EDIT 2) Here is a companion post, which demonstrates the original issue of the disappearing subject line. (/EDIT 2)
(EDIT 3) Issue originally encountered here, for reference. (/EDIT 3)