Tried all that.
Nope, some replies appear in the preview pane, but never appear in the original post. Maybe the preview pane is not accurately showing new replies…but then I don't know why the preview pane is showing those replies at all. Still seems like something is wrong, and it's not because the page needs updating. Even going back an hour later, after the page is filled with 751 posts, and the reply that appeared in the preview pane does NOT appear on the original post. So there is a discrepancy between replies that appear in the preview pane, and then replies that (never) appear on the original post.