Well, well. I suppose it keeps things interesting but believe it or not this is a first! I have never seen posts disappear before and there do not seem to be any clues to be gleaned from the html. So – we need to work out at what point it is vanishing so we can narrow it all down a bit.
To start off – could you go to the forum admin > Toolbox > Data Inspector. See the top right section – Topic View Data. Can you turn on the option to display spThisPost. You – using the same credentials you are currently using in the admin – will be the only person to see this by the way.
Now run the forum – in fact that page you sent me would be good as it just has the one post on it. What you should see is the raw data before it is processed for display. Can you see the post content showing up. It will be rough and unformatted but is it there?
And there is one question I failed to ask at the top of this thread. They clearly used to show up so what has changed? WP Theme? WP Plugin? Recent update to something?