That's apparently a feature of the FR server when posting in non-HTML mode. It automatically finds strings quoted with 7-bit ASCII straight quotes and "fixes" them to use curly quotes.
The result is fine and dandy for the first iteration. But, if somebody quotes the smart-quoted post, they run into the bug, and the result is garbage.
OK, test x: Here are some “quotes” , but this time I’m going to preview.
It’s the preview code, I think. That last test was previewed, inserted smart quotes, and worked.