I concur that the postings always seem to happen. So a screen refresh is the culprit? What's the root of that problem? I'm reluctant to say it's on the users' end (ISP?) since personally, the problem did not exist prior to the FTM hosting change.
FWIW my personal guess is the problem is in the database server. Either a delay in acknowledging the database update or sending the new record back to UBB Threads for display. But it could be on UBB Threads end or it could be a compatibility issue between UBB Threads 7.5.8 and the database server software.
There are too many people, networks, browsers, and browser versions affected for the problem to be local to the user's computers. It has to be something to do with the hosting site — which would include their servers.