Straight quotes were converted to curly either during posting or upon retrieval from database for display.
Posting of curly quotes is not handled.
What happens if we use the HTML ASCII codes for quotes?
(Ampersand+pound sign+34+semi-colon)= "
(Ampersand+pound sign+39+semi-colon)= '
"double quote"
'single quote'
An interesting fact is that, if you post straight quotes (7-bit ASCII) in a non-HTML post, the server converts them to curly quotes (unicode, requiring three bytes of UTF-8 each). Your post displays OK, and you probably don't even notice unless your font is large or your eyes good.
But, then, if somebody quotes your post in a reply, your curlies are replaced with garbage, due to the current bug (unless your respondent edits them to straight quotes or replaces them with appropriate HTML entities (not going to happen)).