Scheduling and managing events using Outlook.com calendar
I shutdown my computer to test how Outlook.com would respond when I edited an Appointment using my Apple's Calendar App on my iPhone. Outlook.com converted the Appointment to a Meeting when I saved the edit. This point towards problem being Outlook.com, not Outlook Desktop App on computer. Below is link to instructions for re-installing Outlook if you want to give it a try. Remember to post results to this thread if you re-install Outlook Desktop App.
REINSTALLING OUTLOOK
Thanks MG for the links and this test result. I get similar and might extract my Outlook.com account from Outlook 2016 PC app first and try that. If that doesn't work, I'll delete and re-install the Outlook 2016 PC app.
Interestingly there doesn't seem to be a problem with creating and editing in the Windows 10 Calendar app. More evidence that it's an Office Outlook 2016 problem.