Using classic Outlook for Windows in business environments
A solution to the syncing issue was given on the 5th page of this thread by Bob Plantz: select two or more messages, right-click and mark them as unread, then right-click on them and mark them as read. A tad clumsy, but it did seem to work.
Another related problem with New Outlook is that it currently does not support setting up rules to manage mail from Gmail and other third party accounts. That was enough to send me back to Old Outlook.