Apple Mail Not Displaying Email Contents

Every month I create a new local Mailbox which I use to manually file emails I've dealt with. Five days ago emails with attachments in these mailboxes stopped displaying. I get no preview of their first couple of lines under the email title and the preview pane remains blank.

Emails prior to this date are unaffected unless I move them to another mailbox and then they no longer load /display. I'm using Apple Mail on Catalina for my email and everything comes in through an Exchange server.

This is not an issue with an individual Mailbox, all now behave in the same manner. Moving an email back to the Inbox leaves it still unable to load. As a test I attempted to rebuild a mailbox containing a few affected emails and the contents vanished and the bug persists. Safe mode makes no difference.

Any guidance or suggestions would be welcomed as so far I've drawn a blank.