Exchange emails marked as read remain in Apple Mail's Unread box on Mac mini

On my new Mac mini M6 Exchange emails are being marked as read in Apple mail but they are not leaving the Unread box. I have tried creating another smart box with Unread as the only condition, rebuilt the index, deleted and added the account back thru Exchange. The box clears non-Exchange emails. To clear the box, I must select the messages, mark them as read then mark them as unread. What else can I do?