Using older versions of Outlook for Mac for personal email and calendar
Have been working through this for the last month. First solution was to delete and re-add the accounts as soon as the error was found. Just found it again, and before going through process above, restarted the Mac and calendar events are showing up in Outlook, iphone calendar, and Mac calendar as they should.
For now, restart seems to be the simplest workaround until a permanent fix/patch is released