Using Classic Outlook on Windows for personal email, calendar, and contact management
We posted a known issue article to track status, https://support.office.com/en-us/article/can-t-edit-the-notes-field-for-a-contact-fb8394ce-04ce-48b5-bae4-be46f77f10fe?ui=en-US&rs=en-US&ad=US.
The Outlook Team found the root cause of the issue. It has to do with the way Outlook handles that field if the contact has the PR_TRANSPORT_MESSAGE_HEADERS property. The only workaround for existing contacts today is to use MFCMAPIto remove the PR_TRANSPORT_MESSAGE_HEADERS property.
As part of the investigation the product team is trying to understand what could be setting that property. Maybe it is an add-in or other solution. If anyone sees the message header being added and knows what it setting it please let me know.