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.