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Did anyone end up getting a solution on this issue? I have one person in the office that has the same thing, and I haven't been able to recreate.

We fixed this (perhaps by fluke!). We migrated the mailbox in question from Exchange 2010 to Exchange 2013 and the issue simply went away - no idea why but I'm so glad it did!
 
I get the same thing. It is NOT however only limited to new appointments created with the same user's iphone. ANY appointments that he is invited to = not created on his own Mac 2016 Outlook - the time zone is set to UTC.
It gives the warning, but if he wanted to change it, it would be all messed up unless he goes in and changes the Time Zone on the calendar entry.

Another clue that might help someone. Every time I go into his Date/Time settings on the System Preferences of his MacOS, Outlook will pop up a message saying "The time zone setting for your computer has changed. Would you like to update the time zone setting in Outlook?" with an update and Don't Update buttons. But - I haven't changed the time zone.. I just opened up the preferences.


Have you tried changing the timezone to manual (unchecked "set time zone to automatically using current location) under System Preferences > Date & Time > Time Zone. Select your time zone and closest city.
 
Just a quick note to let people know what worked for us.

Similar issue, and when we would change the time zone in Outlook 2011 (14.7.1), on OS 10.11.6, from EST to MST, it would change back to EST. None of the suggestions above helped.

We have an Exchange server (I don't know which version), and when I went to our webmail and changed the default time zone from EST to MST, it allowed us to save the setting back in Outlook.
 
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