Sumas, is it possible that your disappearing posts have been on Page Twos, or Threes, of threads, and then when you went back to check them you were on Page One?
That would be one of my dislikes about the new format. The old format took you directly to the most current page of a thread, with any previous pages history-linked above. This format takes you to the original page, with any later pages linked. To me, that is backwards, and it could cause a reply of yours to seemingly disappear; you're back on Page One, which would not have happened, before.