I just compared the addpost.inc.php script file (the one which handles replies, including the Quick Reply) from 7.5.6 Patch 2 to 7.5.8 (which I encourage you to do yourself as well) and they files haven't changed in a way that would affect this behavior (in fact the only thing which has changed is the php compatibility fix, the header comments, and the removal of a variable for image processing for gallery forums).

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