Using Classic Outlook on Windows for personal email, calendar, and contact management
Richard,
Thanks for installing the Beta channel to test. Since you can still repro with the fixed build please submit a ticket for the team to investigate it further? In Outlook click Help, Contact Support and submit the ticket there. The Contact Support option submits a ticket to the Outlook Team and is enabled in Insider builds. Please let me know if you are able to submit the ticket so I can watch for it.
From my understanding of the fix it should enable Outlook to send even with the prior emails that exhibited the behavior. Since you still repro it would be good to have the Outlook Team review the email you are seeing the issue with.